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12c Asynchronous Global Index Maintenance Part III (Re-Make/Re-Model) August 7, 2013

Posted by Richard Foote in 12c, Asynchronous Global Index Maintenance, Global Indexes, Oracle Indexes, Partitioning, Unique Indexes.
3 comments

As I discussed previously in Part I, the space occupied by orphaned row entries associated with asynchronously maintained global indexes is not automatically reclaimed by subsequent DML operations within the index. Hence the need to clean out these orphaned index entries via the various options discussed in Part II.

However, a good question by Jason Bucata asked what about Unique indexes. “If the index entries aren’t marked deleted but are truly still “there” in the structure, does that mean you can’t use this feature if any global indexes are unique” ?

So now I need a Part III to answer this question :)

So same demo setup as before but this time with a Unique index on the ID column:

SQL> create table muse (id number, code number, name varchar2(30)) partition by range (id)
(partition muse1 values less than (1000001), partition muse2 values less than (2000001), partition muse3 values less than (maxvalue));

Table created.

SQL> insert into muse select rownum, mod(rownum,100000), 'DAVID BOWIE' from dual connect by level <= 3000000;

3000000 rows created.

SQL> commit;

Commit complete.

SQL> exec dbms_stats.gather_table_stats(ownname=>user, tabname=>'MUSE', estimate_percent=>null, method_opt=>'FOR ALL COLUMNS SIZE 1');

PL/SQL procedure successfully completed.

SQL> create unique index muse_id_i on muse(id);

Index created.

Let’s now drop a table partition and confirm we indeed do have orphaned unique index entries:

SQL> alter table muse drop partition muse1 update global indexes;

Table altered.

SQL> select index_name, num_rows, s.blocks, leaf_blocks, status, orphaned_entries
from dba_indexes i, dba_segments s where i.index_name = s.segment_name and table_name='MUSE';

INDEX_NAME       NUM_ROWS     BLOCKS LEAF_BLOCKS STATUS   ORP
-------------- ---------- ---------- ----------- -------- ---
MUSE_ID_I         3000000      11264        8216 VALID    YES

SQL> analyze index muse_id_i validate structure;

Index analyzed.

SQL> select name, blocks, lf_rows, del_lf_rows from index_stats;

NAME                     BLOCKS    LF_ROWS DEL_LF_ROWS
-------------------- ---------- ---------- -----------
MUSE_ID_I                  9216    3000000     1000000

If we take a look at a partial block dump of the first (left-most) index leaf block at this stage:

Block header dump: 0x018076dc
Object id on Block? Y
seg/obj: 0x16c11 csc: 0×00.29239b itc: 2 flg: E typ: 2 – INDEX
brn: 0 bdba: 0x18076d8 ver: 0×01 opc: 0
inc: 0 exflg: 0

Itl Xid Uba Flag Lck Scn/Fsc
0×01 0×0000.000.00000000 0×00000000.0000.00 —- 0 fsc 0×0000.00000000
0×02 0xffff.000.00000000 0×00000000.0000.00 C— 0 scn 0×0000.0029239b
Leaf block dump
===============
header address 360728164=0×15804664
kdxcolev 0
KDXCOLEV Flags = – - -
kdxcolok 0
kdxcoopc 0×80: opcode=0: iot flags=— is converted=Y
kdxconco 1
kdxcosdc 0
kdxconro 404
kdxcofbo 844=0x34c
kdxcofeo 1675=0x68b
kdxcoavs 831
kdxlespl 0
kdxlende 0
kdxlenxt 25196253=0x18076dd
kdxleprv 0=0×0
kdxledsz 10
kdxlebksz 8036
row#0[8021] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 00
col 0; len 2; (2): c1 02
row#1[8006] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 01
col 0; len 2; (2): c1 03
row#2[7991] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 02
col 0; len 2; (2): c1 04
row#3[7976] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 03
col 0; len 2; (2): c1 05
row#4[7961] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 04
col 0; len 2; (2): c1 06
row#5[7946] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 05
col 0; len 2; (2): c1 07
row#6[7931] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 06
col 0; len 2; (2): c1 08
row#7[7916] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 07
col 0; len 2; (2): c1 09
row#8[7901] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 08
col 0; len 2; (2): c1 0a
row#9[7886] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 09
col 0; len 2; (2): c1 0b
row#10[7871] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 0a
col 0; len 2; (2): c1 0c
row#11[7856] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 0b
col 0; len 2; (2): c1 0d
row#12[7841] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 0c
col 0; len 2; (2): c1 0e
row#13[7826] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 0d
col 0; len 2; (2): c1 0f
row#14[7811] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 0e
col 0; len 2; (2): c1 10
row#15[7796] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 0f
col 0; len 2; (2): c1 11

As discussed previously, the index leaf block remains “untouched” after the drop table partition operation and has no index entries actually marked as “deleted”. However, just take a note of the rowid values of the first 10 rows. I’m now going to reinsert new rows with an ID between 1 and 10 that were previously deleted as part of dropping the table partition …

SQL> insert into muse select rownum, 42, 'ZIGGY STARDUST' from dual connect by level <= 10;

10 rows created.

SQL> commit;

Commit complete.

SQL> analyze index muse_id_i validate structure;

Index analyzed.

SQL> select name, blocks, lf_rows, del_lf_rows from index_stats;

NAME                     BLOCKS    LF_ROWS DEL_LF_ROWS
-------------------- ---------- ---------- -----------
MUSE_ID_I                  9216    3000000      999990

We can see that the number of so-called deleted leaf rows is now only 999990 and has decreased by the 10 rows we’ve inserted.

If we take a look now at the first index leaf block again:

Block header dump: 0x018076dc
Object id on Block? Y
seg/obj: 0x16c11 csc: 0×00.29239b itc: 2 flg: E typ: 2 – INDEX
brn: 0 bdba: 0x18076d8 ver: 0×01 opc: 0
inc: 0 exflg: 0

Itl Xid Uba Flag Lck Scn/Fsc
0×01 0×0000.000.00000000 0×00000000.0000.00 —- 0 fsc 0×0000.00000000
0×02 0×0004.012.0000079d 0x014045c7.0122.44 –U- 10 fsc 0×0000.00292503
Leaf block dump
===============
header address 360612452=0x157e8264
kdxcolev 0
KDXCOLEV Flags = – - -
kdxcolok 0
kdxcoopc 0×80: opcode=0: iot flags=— is converted=Y
kdxconco 1
kdxcosdc 0
kdxconro 404
kdxcofbo 844=0x34c
kdxcofeo 1675=0x68b
kdxcoavs 831
kdxlespl 0
kdxlende 0
kdxlenxt 25196253=0x18076dd
kdxleprv 0=0×0
kdxledsz 10
kdxlebksz 8036
row#0[8021] flag: ——-, lock: 2, len=15, data:(10): 00 01 6c 0f 01 80 91 2a 00 86
col 0; len 2; (2): c1 02
row#1[8006] flag: ——-, lock: 2, len=15, data:(10): 00 01 6c 0f 01 80 91 2a 00 87
col 0; len 2; (2): c1 03
row#2[7991] flag: ——-, lock: 2, len=15, data:(10): 00 01 6c 0f 01 80 91 2a 00 88
col 0; len 2; (2): c1 04
row#3[7976] flag: ——-, lock: 2, len=15, data:(10): 00 01 6c 0f 01 80 91 2a 00 89
col 0; len 2; (2): c1 05
row#4[7961] flag: ——-, lock: 2, len=15, data:(10): 00 01 6c 0f 01 80 91 2a 00 8a
col 0; len 2; (2): c1 06
row#5[7946] flag: ——-, lock: 2, len=15, data:(10): 00 01 6c 0f 01 80 91 2a 00 8b
col 0; len 2; (2): c1 07
row#6[7931] flag: ——-, lock: 2, len=15, data:(10): 00 01 6c 0f 01 80 91 2a 00 8c
col 0; len 2; (2): c1 08
row#7[7916] flag: ——-, lock: 2, len=15, data:(10): 00 01 6c 0f 01 80 91 2a 00 8d
col 0; len 2; (2): c1 09
row#8[7901] flag: ——-, lock: 2, len=15, data:(10): 00 01 6c 0f 01 80 91 2a 00 8e
col 0; len 2; (2): c1 0a
row#9[7886] flag: ——-, lock: 2, len=15, data:(10): 00 01 6c 0f 01 80 91 2a 00 8f
col 0; len 2; (2): c1 0b
row#10[7871] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 0a
col 0; len 2; (2): c1 0c
row#11[7856] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 0b
col 0; len 2; (2): c1 0d
row#12[7841] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 0c
col 0; len 2; (2): c1 0e
row#13[7826] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 0d
col 0; len 2; (2): c1 0f
row#14[7811] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 0e
col 0; len 2; (2): c1 10
row#15[7796] flag: ——-, lock: 0, len=15, data:(10): 00 01 6c 0e 01 80 11 30 00 0f
col 0; len 2; (2): c1 11

We notice that the first 10 index entries now have different rowids from the previous block dump.

So this is an exception to the rule. With a Unique index, Oracle will indeed reuse the storage occupied by the orphaned Unique index entry if the same unique value as an orphaned value is subsequently re-inserted. This is not the case with Non-Unique indexes, even if such Non-Unique indexes are used to police either a PK or Unique Key constraint.

So the new valid index entries and any existing orphaned entries can be read and/or ignored as necessary:

SQL> select * from muse where id between 1 and 100;

ID       CODE NAME
---------- ---------- --------------------
1         42 ZIGGY STARDUST
2         42 ZIGGY STARDUST
3         42 ZIGGY STARDUST
4         42 ZIGGY STARDUST
5         42 ZIGGY STARDUST
6         42 ZIGGY STARDUST
7         42 ZIGGY STARDUST
8         42 ZIGGY STARDUST
9         42 ZIGGY STARDUST
10        42 ZIGGY STARDUST

10 rows selected.

Execution Plan
----------------------------------------------------------

Plan hash value: 2515419874

------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                                  | Name      | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                           |           |     1 |    23 |     4   (0)| 00:00:01 |       |       |
|   1 |  TABLE ACCESS BY GLOBAL INDEX ROWID BATCHED| MUSE      |     1 |    23 |     4   (0)| 00:00:01 |     1 |     1 |
|*  2 |   INDEX RANGE SCAN                         | MUSE_ID_I |   100 |       |     3   (0)| 00:00:01 |       |       |
------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - access("ID">=1 AND "ID"<=100)

filter(TBL$OR$IDX$PART$NUM("MUSE",0,8,0,"MUSE".ROWID)=1)

Statistics
----------------------------------------------------------

5  recursive calls
0  db block gets
7  consistent gets
0  physical reads
0  redo size
974  bytes sent via SQL*Net to client
544  bytes received via SQL*Net from client
2  SQL*Net roundtrips to/from client
1  sorts (memory)
0  sorts (disk)
10  rows processed

However, if new unique values are inserted into the table but with ID values that didn’t previously exist:

SQL> insert into muse select rownum+3000000, 42, 'ZIGGY STARDUST'
from dual connect by level <= 1000000;

1000000 rows created.

SQL> commit;

Commit complete.

SQL> analyze index muse_id_i validate structure;

Index analyzed.

SQL> select name, blocks, lf_rows, del_lf_rows from index_stats;

NAME                     BLOCKS    LF_ROWS DEL_LF_ROWS
-------------------- ---------- ---------- -----------
MUSE_ID_I                 11264    4000000      999990

We notice that the number of so-called deleted leaf entries remains the same after inserting the 1M new rows.

So in this scenario, the effectively “empty” leaf blocks containing nothing but orphaned unique index entries are not re-cycled and reused by subsequent index block splits as they would have been had they contained nothing but deleted index entries.

So Unique indexes in the unlikely event that such unique values are subsequently reinserted are an exception to the general rule of orphaned global index entries having to be “cleaned out”.

12c Asynchronous Global Index Maintenance Part II (The Space Between) August 6, 2013

Posted by Richard Foote in 12c, Asynchronous Global Index Maintenance, Coalesce Cleanup, dbms_part.cleanup_gidx, Index Coalesce, Oracle Indexes, Partitioning.
7 comments

In Part I, I discussed how global indexes can now be asynchronously maintained in Oracle 12c when a table partition is dropped or truncated. Basically, when a table partition is dropped/truncated with the UPDATE GLOBAL INDEXES clause, Oracle simply keeps track of the object numbers of those table partitions and ignores any corresponding rowids within the index during subsequent index scans. As such, these table partition operations are very fast and efficient as the global indexes are not actually maintained during the partition operation, but importantly, continue to remain in a usable state.

If we look at a partial 11g global index block dump after dropping a table partition (eg. the MUSE_ID_I in the previous demo):

Block header dump: 0×01028750
Object id on Block? Y
seg/obj: 0x130ac csc: 0×00.3c7323 itc: 2 flg: E typ: 2 – INDEX
brn: 1 bdba: 0×1028748 ver: 0×01 opc: 0
inc: 0 exflg: 0

Itl Xid Uba Flag Lck Scn/Fsc
0×01 0×0000.000.00000000 0×00000000.0000.00 —- 0 fsc 0×0000.00000000
0×02 0×0006.001.00000f91 0x00c03e16.0177.02 —- 378 fsc 0x1c0b.00000000
Leaf block dump
===============
header address 130573412=0x7c86464
kdxcolev 0
KDXCOLEV Flags = – - -
kdxcolok 0
kdxcoopc 0×80: opcode=0: iot flags=— is converted=Y
kdxconco 2
kdxcosdc 0
kdxconro 378
kdxcofbo 792=0×318
kdxcofeo 1613=0x64d
kdxcoavs 821
kdxlespl 0
kdxlende 378
kdxlenxt 16942929=0×1028751
kdxleprv 16942927=0x102874f
kdxledsz 0
kdxlebksz 8036
row#0[8019] flag: —D–, lock: 2, len=17
col 0; len 3; (3): c2 10 13
col 1; len 10; (10): 00 01 30 a9 01 02 ab 34 00 3b
row#1[8002] flag: —D–, lock: 2, len=17
col 0; len 3; (3): c2 10 14
col 1; len 10; (10): 00 01 30 a9 01 02 ab 34 00 3c
row#2[7985] flag: —D--, lock: 2, len=17
col 0; len 3; (3): c2 10 15
col 1; len 10; (10): 00 01 30 a9 01 02 ab 34 00 3d
row#3[7968] flag: —D--, lock: 2, len=17
col 0; len 3; (3): c2 10 16
col 1; len 10; (10): 00 01 30 a9 01 02 ab 34 00 3e
row#4[7951] flag: —D–, lock: 2, len=17
col 0; len 3; (3): c2 10 17
col 1; len 10; (10): 00 01 30 a9 01 02 ab 34 00 3f

….

We notice that all index entries that reference the dropped table partition are marked as deleted. They all have a D (deleted) flag set and have been locked by the drop table partition transaction in ITL slot 2. So prior to Oracle 12c, to update global indexes on the fly was a relatively expensive operation as it required all the associated index entries to be deleted from the global indexes.

However, if we look at a block dump of the same index in an Oracle 12c database following a table partition being dropped:

Block header dump: 0x018000e0
Object id on Block? Y
seg/obj: 0x16bbe csc: 0×00.26ae40 itc: 2 flg: E typ: 2 – INDEX
brn: 1 bdba: 0x18000d8 ver: 0×01 opc: 0
inc: 0 exflg: 0

Itl Xid Uba Flag Lck Scn/Fsc
0×01 0×0000.000.00000000 0×00000000.0000.00 —- 0 fsc 0×0000.00000000
0×02 0xffff.000.00000000 0×00000000.0000.00 C— 0 scn 0×0000.0026ae40
Leaf block dump
===============
header address 364741220=0x15bd8264
kdxcolev 0
KDXCOLEV Flags = – - -
kdxcolok 0
kdxcoopc 0×80: opcode=0: iot flags=— is converted=Y
kdxconco 2
kdxcosdc 0
kdxconro 378
kdxcofbo 792=0×318
kdxcofeo 1613=0x64d
kdxcoavs 821
kdxlespl 0
kdxlende 0
kdxlenxt 25166049=0x18000e1
kdxleprv 25166047=0x18000df
kdxledsz 0
kdxlebksz 8036
row#0[8019] flag: ——-, lock: 0, len=17
col 0; len 3; (3): c2 10 13
col 1; len 10; (10): 00 01 6b bb 01 80 04 71 00 3b
row#1[8002] flag: ——-, lock: 0, len=17
col 0; len 3; (3): c2 10 14
col 1; len 10; (10): 00 01 6b bb 01 80 04 71 00 3c
row#2[7985] flag: ——-, lock: 0, len=17
col 0; len 3; (3): c2 10 15
col 1; len 10; (10): 00 01 6b bb 01 80 04 71 00 3d
row#3[7968] flag: ——-, lock: 0, len=17
col 0; len 3; (3): c2 10 16
col 1; len 10; (10): 00 01 6b bb 01 80 04 71 00 3e
row#4[7951] flag: ——-, lock: 0, len=17
col 0; len 3; (3): c2 10 17
col 1; len 10; (10): 00 01 6b bb 01 80 04 71 00 3f

We notice there are no deleted index entries, the index remains totally untouched by the drop table partition operation. So the good news is that dropping/truncating a table partition while updating global indexes is extremely fast and efficient while the indexes remain hunky dory as subsequent index range scans can ignore any rowids that don’t reference existing table partitions of interest.

However, the bad news is that during subsequent index DML operations, Oracle does not know which index entries are valid and which are not and so the space used by these “orphaned” index entries can not be automatically reclaimed and reused as it can with conventionally deleted index entries. Therefore, we need some other way to clean out the orphaned index entries.

There are a number of possible ways to do this. One way is to simply rebuild the global index (or index partition):

SQL> alter index muse_code_i rebuild partition code_p1;

Index altered.

SQL> select index_name, null partition_name, orphaned_entries, num_rows, s.blocks, leaf_blocks, status
from dba_indexes i, dba_segments s where i.index_name = s.segment_name and table_name='MUSE' and partitioned = 'NO'
union
select index_name, i.partition_name, orphaned_entries, num_rows, s.blocks, leaf_blocks, status
from dba_ind_partitions i, dba_segments s where i.partition_name = s.partition_name and index_name like 'MUSE%';

INDEX_NAME   PARTITION_NAME  ORP  NUM_ROWS     BLOCKS LEAF_BLOCKS STATUS
------------ --------------- ------------- ---------- ----------- --------
MUSE_CODE_I  CODE_P1         NO    1000000       2944        2758 USABLE
MUSE_CODE_I  CODE_P2         YES   1000000       4352        4177 USABLE
MUSE_ID_I                    YES   2000000       9216        5849 VALID

Effective, but relatively expensive as this requires the entire index structure to be rebuilt from scratch. Depending on the scale and distribution of the orphaned index entries, another possibly cheaper alternative is to use the new CLEANUP coalesce clause:

SQL> alter index muse_id_i coalesce cleanup;

Index altered.

SQL> exec dbms_stats.gather_index_stats(ownname=>user, indname=>'MUSE_ID_I', estimate_percent=>null);

PL/SQL procedure successfully completed.

SQL> select index_name, null partition_name, orphaned_entries, num_rows, s.blocks, leaf_blocks, status
from dba_indexes i, dba_segments s where i.index_name = s.segment_name and table_name='MUSE' and partitioned = 'NO'
union
select index_name, i.partition_name, orphaned_entries, num_rows, s.blocks, leaf_blocks, status
from dba_ind_partitions i, dba_segments s where i.partition_name = s.partition_name and index_name like 'MUSE%';

INDEX_NAME   PARTITION_NAME  ORP  NUM_ROWS     BLOCKS LEAF_BLOCKS STATUS
------------ --------------- --- --------- ---------- ----------- --------
MUSE_CODE_I  CODE_P1         NO    1000000       4224        4137 USABLE
MUSE_CODE_I  CODE_P2         YES   1000000       4352        4177 USABLE
MUSE_ID_I                    NO    2000000       9216        5849 VALID

SQL> analyze index muse_id_i validate structure;

Index analyzed.

SQL> select name, lf_rows, del_lf_rows from index_stats;

NAME            LF_ROWS DEL_LF_ROWS
------------ ---------- -----------
MUSE_ID_I       2000000           0

This visits each index leak block and removes all the orphaned index entries as part of the coalesce process. Note this is a more “powerful” version of coalesce as a standard coalesce is not aware of orphaned index entries and will only coalesce the index without actually removing the orphaned index.

Yet another possible option is to simply wait for the PMO_DEFERRED_GIDX_MAINT_JOB job to run (scheduled by default during the 2am maintenance window) to clean out orphaned index entries from all currently impacted global indexes. Yet another alternative is to manually run the dbms_part.cleanup_gidx procedure which is in turn called by this job:

SQL> exec dbms_part.cleanup_gidx;

PL/SQL procedure successfully completed.

SQL> select index_name, null partition_name, orphaned_entries, num_rows, s.blocks, leaf_blocks, status
from dba_indexes i, dba_segments s where i.index_name = s.segment_name and table_name='MUSE' and partitioned = 'NO'
union
select index_name, i.partition_name, orphaned_entries, num_rows, s.blocks, leaf_blocks, status
from dba_ind_partitions i, dba_segments s where i.partition_name = s.partition_name and index_name like 'MUSE%';

INDEX_NAME   PARTITION_NAME  ORP   NUM_ROWS     BLOCKS LEAF_BLOCKS STATUS
------------ --------------- --- ---------- ---------- ----------- --------
MUSE_CODE_I  CODE_P1         NO     1000000       2944        2758 USABLE
MUSE_CODE_I  CODE_P2         NO     1000000       4352        4177 USABLE
MUSE_ID_I                    NO     2000000       9216        5849 VALID

We notice the last index partition has now been cleaned out and no longer has orphaned index entries.

So with the new asynchronous global index maintenance capabilities of the Oracle 12c database, we can perform a much faster and more efficient drop/truncate table partition operation while keeping our global indexes in a usable state and leave the tidying up of the resultant orphaned index entries to another time and method of our convenience.

12c Asynchronous Global Index Maintenance Part I (Where Are We Now ?) August 2, 2013

Posted by Richard Foote in 12c, Asynchronous Global Index Maintenance, Oracle Indexes, Partitioning.
4 comments

I previously looked at how global index maintenance was performed when dropping a table partition prior to Oracle Database 12c. Let’s see how things have now changed since the introduction of 12c.

Let’s start by creating the same partitioned table and global indexes as previously:

SQL> create table muse (id number, code number, name varchar2(30)) partition by range (id) (partition muse1 values less than (1000001), partition muse2 values less than (2000001), partition muse3 values less than (maxvalue));

Table created.

SQL> insert into muse select rownum, mod(rownum,100000), 'DAVID BOWIE' from dual connect by level <= 3000000;

3000000 rows created.

SQL> commit;

Commit complete.

SQL> create index muse_id_i on muse(id);

Index created.

SQL> create index muse_code_i on muse(code) global partition by range(code)(partition code_p1 values less than (50000), partition code_p2 values less than (maxvalue));

Index created.

SQL> exec dbms_stats.gather_table_stats(ownname=>user, tabname=>'MUSE', cascade=>true, estimate_percent=>null, method_opt=>'FOR ALL COLUMNS SIZE 1');

PL/SQL procedure successfully completed.

If we look at the current state of affairs, all is currently hunky dory:

SQL> select index_name, null partition_name, num_rows, s.blocks, leaf_blocks, status, orphaned_entries from dba_indexes i, dba_segments s where i.index_name = s.segment_name and table_name='MUSE' and partitioned = 'NO'
union select index_name, i.partition_name, num_rows, s.blocks, leaf_blocks, status, orphaned_entries from dba_ind_partitions i, dba_segments s where i.partition_name = s.partition_name and index_name like 'MUSE%';

INDEX_NAME   PARTITION_NAME    NUM_ROWS     BLOCKS LEAF_BLOCKS STATUS   ORP
------------ --------------- ---------- ---------- ----------- -------- ---
MUSE_CODE_I  CODE_P1            1500000       4224        4137 USABLE   NO
MUSE_CODE_I  CODE_P2            1500000       4352        4177 USABLE   NO
MUSE_ID_I                       3000000       9216        8633 VALID    NO

However, a difference to note here is a new data dictionary column called ORPHANED_ENTRIES which denotes whether the index currently has any orphaned index entries. What are these ? We shall see …

Let’s see how expensive it is to now drop the same table partition while updating the global indexes:

SQL> select n.name, s.value from v$sesstat s, v$statname n where s.statistic# = n.statistic# and (n.name = 'redo size' or n.name = 'db block gets') and s.sid=7;

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
db block gets                                                        129249
redo size                                                         105069544

SQL> alter table muse drop partition muse1 update global indexes;

Table altered.

Elapsed: 00:00:00.76

SQL> select n.name, s.value from v$sesstat s, v$statname n where s.statistic# = n.statistic# and (n.name = 'redo size' or n.name = 'db block gets') and s.sid=7;

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
db block gets                                                        129314
redo size                                                         105083724

As we can see, this is significantly different than before when this was a relatively slow and expensive exercise. At just 65 block gets and only 14180 bytes of redo, this is now about the same cost as dropping the partition without updating the global indexes. How can this be ?

If we now look at the status of our global indexes:

SQL> exec dbms_stats.gather_table_stats(ownname=>user, tabname=>'MUSE', cascade=>true, estimate_percent=>null, method_opt=>'FOR ALL COLUMNS SIZE 1');

PL/SQL procedure successfully completed.

SQL> select index_name, null partition_name, num_rows, s.blocks, leaf_blocks, status, orphaned_entries
from dba_indexes i, dba_segments s where i.index_name = s.segment_name and table_name='MUSE' and partitioned = 'NO'
union
select index_name, i.partition_name, num_rows, s.blocks, leaf_blocks, status, orphaned_entries
from dba_ind_partitions i, dba_segments s where i.partition_name = s.partition_name and index_name like 'MUSE%';

INDEX_NAME   PARTITION_NAME    NUM_ROWS     BLOCKS LEAF_BLOCKS STATUS   ORP
------------ --------------- ---------- ---------- ----------- -------- ---
MUSE_CODE_I  CODE_P1            1000000       4224        4137 USABLE   YES
MUSE_CODE_I  CODE_P2            1000000       4352        4177 USABLE   YES
MUSE_ID_I                       2000000       9216        5849 VALID    YES

We notice that indeed, the index entries have been reduced (for example, only 2M index entries now in MUSE_ID_I instead of 3M) as if the indexes have been updated. However, we also notice that although the indexes are both marked as now having orphaned entries, they’re still in a USABLE state.

Basically, when dropping (or truncating) a table partition, Oracle in 12c now “postpones” the actual removal of the global index entries associated with the dropped/truncated partition. This can now be performed asynchronously at a time of our choosing. So it’s therefore now very quick and very cheap to update these global indexes on the fly.

However, most importantly, the indexes are still usable and can be guaranteed to return the correct results, ignoring any orphaned entires as required. These can be easily ignored as they all have an object number in the index entry rowids associated with the dropped table partition object and not the table partition(s) of interest as required by the queries.

So if we now select values via the ID column index that only spans data in the dropped table partition:

SQL> select * from muse where id between 42 and 420;

no rows selected

Execution Plan
----------------------------------------------------------
Plan hash value: 2515419874
------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                                  | Name      | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                           |           |     1 |    23 |     4   (0)| 00:00:01 |       |       |
|   1 |  TABLE ACCESS BY GLOBAL INDEX ROWID BATCHED| MUSE      |     1 |    23 |     4   (0)| 00:00:01 |     1 |     1 |
|*  2 |   INDEX RANGE SCAN                         | MUSE_ID_I |     1 |       |     3   (0)| 00:00:01 |       |       |
------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - access("ID">=42 AND "ID"<=420)

filter(TBL$OR$IDX$PART$NUM("MUSE",0,8,0,"MUSE".ROWID)=1)

Statistics
----------------------------------------------------------

0  recursive calls
0  db block gets
4  consistent gets
0  physical reads
0  redo size
470  bytes sent via SQL*Net to client
532  bytes received via SQL*Net from client
1  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
0  rows processed

We notice that quite correctly, no rows are now returned.

The partitioned global index on the CODE column likewise only returns valid data when accessed:

SQL> select * from muse where code=42;

ID       CODE NAME
---------- ---------- ------------------------------
1000042         42 DAVID BOWIE
1100042         42 DAVID BOWIE
1200042         42 DAVID BOWIE
1300042         42 DAVID BOWIE
1400042         42 DAVID BOWIE
1500042         42 DAVID BOWIE
1700042         42 DAVID BOWIE
1600042         42 DAVID BOWIE
1900042         42 DAVID BOWIE
1800042         42 DAVID BOWIE
2000042         42 DAVID BOWIE
2100042         42 DAVID BOWIE
2200042         42 DAVID BOWIE
2300042         42 DAVID BOWIE
2400042         42 DAVID BOWIE
2500042         42 DAVID BOWIE
2600042         42 DAVID BOWIE
2700042         42 DAVID BOWIE
2900042         42 DAVID BOWIE
2800042         42 DAVID BOWIE

20 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 4070098220
---------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                                   | Name        | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
---------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                            |             |    20 |   460 |    24   (0)| 00:00:01 |       |       |
|   1 |  PARTITION RANGE SINGLE                     |             |    20 |   460 |    24   (0)| 00:00:01 |     1 |     1 |
|   2 |   TABLE ACCESS BY GLOBAL INDEX ROWID BATCHED| MUSE        |    20 |   460 |    24   (0)| 00:00:01 | ROWID | ROWID |
|*  3 |    INDEX RANGE SCAN                         | MUSE_CODE_I |    20 |       |     3   (0)| 00:00:01 |     1 |     1 |
---------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

3 - access("CODE"=42)

filter(TBL$OR$IDX$PART$NUM("MUSE",0,8,0,"MUSE".ROWID)=1)

Statistics

----------------------------------------------------------
1  recursive calls
0  db block gets
25  consistent gets
2  physical reads
0  redo size
1147  bytes sent via SQL*Net to client
554  bytes received via SQL*Net from client
3  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
20  rows processed

As we can see, only valid data belonging to the non-dropped partitions are returned via the index, even though the index has orphaned index entries that reference the dropped table partition.

If we look at the INDEX_STATS of these indexes, we notice at one level that the orphaned index entries are counted as if they’re deleted entries:

SQL> analyze index muse_id_i validate structure;

Index analyzed.

SQL> select name, lf_rows, del_lf_rows from index_stats;

NAME            LF_ROWS DEL_LF_ROWS
------------ ---------- -----------
MUSE_ID_I       3000000     1000000

We see that the index statistics is indicating that there are 1M so-called deleted index entries. The validation process is ensuring that the orphaned index entries only reference partitions that indeed no longer exist and counts such entries as deleted ones.

So it currently looks we’ve got the best of both worlds here. We effectively get the same performance during the drop table partition operation as if we don’t maintain the global indexes but get the same index availability and subsequent query performance as if we do. So what’s the catch ?

Well, very importantly, unlike actual deleted index entries, they are not readily removed and their space reused by subsequent DML activities within the leaf blocks. In fact, these orphaned index entries can even “get in the way” as we see here when we attempt to reinsert the same data back into table:

SQL> insert into muse select rownum, mod(rownum,100000), 'DAVID BOWIE' from dual connect by level <= 1000000;

1000000 rows created.

Elapsed: 00:03:56.52

Execution Plan
----------------------------------------------------------

Plan hash value: 1731520519
-------------------------------------------------------------------------------
| Id  | Operation                      | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | INSERT STATEMENT               |      |     1 |     2   (0)| 00:00:01 |
|   1 |  LOAD TABLE CONVENTIONAL       | MUSE |       |            |          |
|   2 |   COUNT                        |      |       |            |          |
|*  3 |    CONNECT BY WITHOUT FILTERING|      |       |            |          |
|   4 |     FAST DUAL                  |      |     1 |     2   (0)| 00:00:01 |
-------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

3 - filter(LEVEL<=1000000)

Statistics
----------------------------------------------------------

700  recursive calls
758362  db block gets
53062  consistent gets
5069  physical reads
355165692  redo size
869  bytes sent via SQL*Net to client
903  bytes received via SQL*Net from client
3  SQL*Net roundtrips to/from client
75  sorts (memory)
0  sorts (disk)
1000000  rows processed

SQL> commit;

Commit complete.

This is notably slower and more expensive than if the index entries had actually been deleted because Oracle is not able to simply identify and overwrite the orphaned index entries during DML operations as they’re not physically marked as deleted. If we look at the INDEX_STATS after inserting these new rows:

SQL> analyze index muse_id_i validate structure;

Index analyzed.

SQL> select name, blocks, lf_rows, del_lf_rows from index_stats;

NAME                BLOCKS    LF_ROWS DEL_LF_ROWS
--------------- ---------- ---------- -----------
MUSE_ID_I            12288    4000000     1000000

SQL> analyze index muse_code_i validate structure;

Index analyzed.

SQL> select name, blocks, lf_rows, del_lf_rows from index_stats;

NAME                BLOCKS    LF_ROWS DEL_LF_ROWS
--------------- ---------- ---------- -----------
MUSE_CODE_I           9216    2000000      500000

We notice that unlike actual deleted index entries in which all the deleted space would have simply have been reused, we see instead  that none of the space occupied by the orphaned rows has been reused. This in the end means accessing more index blocks, potentially performing more block splits, more newer blocks having to be generated and overall more work having to be performed than would have been necessary if they had just been plain deleted index entries.

So how we actually get rid of these orphaned index entries ? I look at a number of different techniques we can use in Part II. And yes, good old block dumps are on their way as well :)

Upcoming Oracle Events in Canberra (The Show Must Go On) July 19, 2013

Posted by Richard Foote in 12c, INSYNC13, Oracle.
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For those of you who live locally in the Canberra region, there are a couple of key Oracle events coming up.

Oracle Database 12c Launch

Date: Friday, July 26 2013

Time: 8:30 AM – 1:00 PM

Where: Hyatt Hotel

120 Commonwealth Ave

Yarralumla, Canberra

Details and Registration

I’ll be presenting “Simplify Consolidation with Oracle Database 12c”.

—-

AUSOUG INSYNC13 Conference

Date: 27-28 August 2013

Where: Crown Plaza Hotel

1 Binara Street

Canberra City

Program Details and Registration

I’ll be presenting “New Indexing Features Introduced in Oracle Database 12c”.

See you there :)

12c Partial Indexes For Partitioned Tables Part II (Vanishing Act) July 12, 2013

Posted by Richard Foote in 12c, Local Indexes, Oracle Indexes, Partial Indexes, Partitioning.
3 comments

In Partial Indexes Part I, we looked at how it was possible with the 12c database  to create a Partial Index based on data from only selected table partitions. The resultant Partial Index can be either a Global or Local Index.

In Part I, we only really looked at Global Indexes, so let’s look at a Local Index example. Using the same Partitioned Table example as before:

SQL> create table pink_floyd (id number, status varchar2(6), name varchar2(30))
indexing off
partition by range (id)
(partition pf1 values less than (1000001),
partition pf2 values less than (2000001) indexing off,
partition pf3 values less than (maxvalue) indexing on);
Table created.

This time, we’ll create a Local Partial Index:

SQL> create index pink_floyd_status_i on pink_floyd(status)
local indexing partial;

Index created.

If we look at the details of the resultant Local Index:

SQL> select index_name, partition_name, num_rows, status, leaf_blocks from dba_ind_partitions where index_name = 'PINK_FLOYD_STATUS_I';

INDEX_NAME           PARTITION_NAME    NUM_ROWS STATUS   LEAF_BLOCKS
-------------------- --------------- ---------- -------- -----------
PINK_FLOYD_STATUS_I  PK1                      0 UNUSABLE           0
PINK_FLOYD_STATUS_I  PK2                      0 UNUSABLE           0
PINK_FLOYD_STATUS_I  PK3                1000000 USABLE          2513

We can see that for those table partitions with INDEXING OFF, the associated Local Indexes have simply been made UNUSABLE. Since Unusable Indexes consume no storage, there is effectively no corresponding index segment for these index partitions.

For the one and only PK3 table partition with INDEXING ON, its associated Local Index has been created as normal. So the end result is very similar to the previous Global Index example, only those rows from the table partitions with the INDEXING ON property are effectively being indexed.

There is one scenario in which the creation of a Partial Index is not permitted, that is in the creation of a Unique Index or a Non-Unique Index to police a Primary Key or Unique Key constraint. Some examples:

SQL> create unique index pink_floyd_id_i on pink_floyd(id)
indexing partial;
create unique index pink_floyd_id_i on pink_floyd(id) indexing partial
*
ERROR at line 1:

ORA-14226: unique index may not be PARTIAL

SQL> alter table pink_floyd add constraint pink_floyd_pk primary key(id)
using index (create index pink_floyd_id_i on pink_floyd(id) indexing partial);
alter table pink_floyd add constraint pink_floyd_pk primary key(id) using index
(create index pink_floyd_id_i on pink_floyd(id) indexing partial)
*
ERROR at line 1:
ORA-14196: Specified index cannot be used to enforce the constraint.

SQL> create index pink_floyd_id_i on pink_floyd(id) indexing partial;

Index created.

SQL> alter table pink_floyd add primary key(id);
alter table pink_floyd add primary key(id)
*
ERROR at line 1:

ORA-01408: such column list already indexed

It clearly doesn’t make sense to create a Partial Unique Index or on a Non-Unique Index policing a PK or Unique Key constraint as it would be impossible to use such an index to guarantee the required unique property. With missing index entries associated with non-indexed partitions, how can Oracle determine whether a value from new row already exists or not ? It can’t and hence Oracle doesn’t permit the creation of such a Partial Index.

Partial Indexes can potentially be extremely useful in reducing unnecessary storage requirements, reducing index maintenance overheads and in improving performance by reducing index block accesses.

But they’re only useful (possible) with Partitioned Tables.

I’ll next look at another cool index improvement introduced with the Oracle 12c Database that’s associated with Partitioning, Asynchronous Global Index Maintenance

12c Partial Indexes For Partitioned Tables Part I (Ignoreland) July 8, 2013

Posted by Richard Foote in 12c, Oracle Indexes, Partial Indexes, Partitioning.
8 comments

In my opinion, one of the unsung “Heroes” of the new 12c Oracle database are improvements associated with Partitioning. There are lots of really nice capabilities introduced in the 12c database that significantly improves the manageability of partitioned environments.

One of those new capabilities is the ability to now create both local and (importantly) global  indexes on only a subset of partitions within a partitioned table. This provides us with the flexibility to say only create partitions with data that would make sense to index, to not index current partitions where perhaps data insert performance is paramount, etc. Additionally and just as importantly, the CBO is aware of the indexing characteristics of individual partitions and can access partitions in differing manners accordingly.

To illustrate, a simple little demo as usual :) Firstly, I’ll create a partitioned table with the new INDEXING clause:

SQL> create table pink_floyd (id number, status varchar2(6), name varchar2(30))
INDEXING OFF
partition by range (id)
(partition pf1 values less than (1000001),
partition pf2 values less than (2000001) INDEXING OFF,
partition pf3 values less than (maxvalue) INDEXING ON);
Table created.

The INDEXING clause determines whether or not the partition is to be indexed. It can be set at the table level and so set the default behaviour for the table or at the individual partition/subpartition level.

In the above example, I’ve set INDEXING OFF at the table level and so indexing by default is not be enabled for the table partitions. Therefore the PF1 partition is not indexed by default. The PF2 partition is explicitly set to also not be indexed but the PF3 index is explicitly set (INDEXING ON) to enable indexing and so override the table level default.

Let’s now populate the table with some basic data:

SQL> insert into pink_floyd select rownum, 'CLOSED', 'DAVID BOWIE' from dual connect by level <= 3000000;

3000000 rows created.

SQL> commit;

Commit complete.

SQL> update pink_floyd set status = 'OPEN' where id > 2000000 and mod(id,10000)=0;

100 rows updated.

SQL> commit;

Commit complete.

Most of the data has a STATUS column value of ‘CLOSED’ but I’ve updated a few rows within just the last partition with a STATUS set to ‘OPEN’.

Let’s now create an index on this STATUS column and collect table statistics:

SQL> create index pink_floyd_status_i on pink_floyd(status);

Index created.

SQL> exec dbms_stats.gather_table_stats(ownname=>user, tabname=>'PINK_FLOYD', estimate_percent=>null, cascade=> true, method_opt=>'FOR ALL COLUMNS SIZE 1 FOR COLUMNS STATUS SIZE 5');

PL/SQL procedure successfully completed.

SQL> select index_name, num_rows, leaf_blocks, indexing from dba_indexes where index_name = 'PINK_FLOYD_STATUS_I';

INDEX_NAME             NUM_ROWS LEAF_BLOCKS INDEXIN
-------------------- ---------- ----------- -------
PINK_FLOYD_STATUS_I     3000000        9203 FULL

By default, an index will include all partitions in a table, regardless of the INDEXING table clause setting. So this index covers all 3M rows in the table and currently has 9203 leaf blocks. The new INDEXING column in DBA_INDEXES shows us that this index is a FULL (non-Partial) index.

We can of course get the data of interest (STATUS = ‘OPEN’) via this index now:

SQL> select * from pink_floyd where status = 'OPEN';

100 rows selected.

Execution Plan
----------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                                  | Name                | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
----------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                           |                     |   964 | 24100 |     4   (0)| 00:00:01 |       |       |
|   1 |  TABLE ACCESS BY GLOBAL INDEX ROWID BATCHED| PINK_FLOYD          |   964 | 24100 |     4   (0)| 00:00:01 | ROWID | ROWID |
|*  2 |   INDEX RANGE SCAN                         | PINK_FLOYD_STATUS_I |   100 |       |     3   (0)| 00:00:01 |       |       |
----------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - access("STATUS"='OPEN')

Statistics
----------------------------------------------------------

0  recursive calls

0  db block gets

110  consistent gets

0  physical reads

0  redo size

4479  bytes sent via SQL*Net to client

609  bytes received via SQL*Net from client

8  SQL*Net roundtrips to/from client

0  sorts (memory)

0  sorts (disk)

100  rows processed

However, we can potentially also run a query based on just the last partition as all the ‘OPEN’ statuses of interest only reside in this last partition:

SQL> select * from pink_floyd where status = 'OPEN' and id > 2000001;

100 rows selected.

Execution Plan
----------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                                  | Name                | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
----------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                           |                     |    99 |  2475 |     4   (0)| 00:00:01 |       |       |
|*  1 |  TABLE ACCESS BY GLOBAL INDEX ROWID BATCHED| PINK_FLOYD          |    99 |  2475 |     4   (0)| 00:00:01 |     3 |     3 |
|*  2 |   INDEX RANGE SCAN                         | PINK_FLOYD_STATUS_I |   100 |       |     3   (0)| 00:00:01 |       |       |
----------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter("ID">2000001)
2 - access("STATUS"='OPEN')

Statistics
----------------------------------------------------------

0  recursive calls

0  db block gets

110  consistent gets

0  physical reads

0  redo size

2787  bytes sent via SQL*Net to client

609  bytes received via SQL*Net from client

8  SQL*Net roundtrips to/from client

0  sorts (memory)

0  sorts (disk)

100  rows processed

Currently however, the index includes data from all table partitions, even though we’re only really interested in using the index to retrieve the less common ‘OPEN’ status that resides in only the last table partition. With 12c, there is now the capability to only index those partitions that are of interest to us, which with proper design can also be implemented such that only those column values of interest are included within an index.

I’m going to drop and create the index as a “Partial” Index:

SQL> drop index pink_floyd_status_i;

Index dropped.

SQL> create index pink_floyd_status_i on pink_floyd(status) indexing partial;

Index created.

SQL> select index_name, num_rows, leaf_blocks, indexing from dba_indexes where index_name = 'PINK_FLOYD_STATUS_I';

INDEX_NAME             NUM_ROWS LEAF_BLOCKS INDEXIN
-------------------- ---------- ----------- -------
PINK_FLOYD_STATUS_I     1000000        3068 PARTIAL

The new INDEXING PARTIAL clause means only those table partitions with INDEXING ON are to be included within the index.

Notice how the index, which is a Global, Non-Partitioned Index, now only has 1M entries (not all 3M as previously) and with 3068 leaf blocks is only 1/3 of what it was previously. The INDEXING column now denotes this as a “Partial” index.

If we run the query again that only explicitly references the last “active” table partition:

SQL> select * from pink_floyd where status = 'OPEN' and id >= 2000001;

100 rows selected.

Execution Plan
----------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                                  | Name                | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
----------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                           |                     |   100 |  2500 |     4   (0)| 00:00:01 |       |       |
|*  1 |  TABLE ACCESS BY GLOBAL INDEX ROWID BATCHED| PINK_FLOYD          |   100 |  2500 |     4   (0)| 00:00:01 |     3 |     3 |
|*  2 |   INDEX RANGE SCAN                         | PINK_FLOYD_STATUS_I |    33 |       |     3   (0)| 00:00:01 |       |       |
----------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter("ID">=2000001)
2 - access("STATUS"='OPEN')

Statistics
----------------------------------------------------------

0  recursive calls

0  db block gets

110  consistent gets

0  physical reads

0  redo size

2787  bytes sent via SQL*Net to client

609  bytes received via SQL*Net from client

8  SQL*Net roundtrips to/from client

0  sorts (memory)

0  sorts (disk)

100  rows processed

We see that the index is used as it was previously. By stating with the ID > 2000001 predicate we’re only interested in data that can only reside in the last table partition, the partition with INDEXING ON, the CBO knows the index can be used to retrieve all the rows of interest. If we know the application will only extract data in this manner, all is well with our smaller, Partial index.

However, if it’s possible within the application to perhaps search for STATUS values from other partitions, that have INDEXING OFF:

SQL> select * from pink_floyd where status = 'OPEN';

100 rows selected.

Execution Plan

----------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                                    | Name                | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop
----------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                             |                     |   100 |  2500 |  2474   (1)| 00:00:01 |       |
|   1 |  VIEW                                        | VW_TE_2             |    99 |  3465 |  2474   (1)| 00:00:01 |       |
|   2 |   UNION-ALL                                  |                     |       |       |            |          |       |
|*  3 |    TABLE ACCESS BY GLOBAL INDEX ROWID BATCHED| PINK_FLOYD          |    33 |   825 |     4   (0)| 00:00:01 | ROWID | ROWID
|*  4 |     INDEX RANGE SCAN                         | PINK_FLOYD_STATUS_I |   100 |       |     3   (0)| 00:00:01 |       |
|   5 |    PARTITION RANGE ITERATOR                  |                     |    66 |  1650 |  2470   (1)| 00:00:01 |     1 |     2
|*  6 |     TABLE ACCESS FULL                        | PINK_FLOYD          |    66 |  1650 |  2470   (1)| 00:00:01 |     1 |     2
----------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

3 - filter("PINK_FLOYD"."ID">=2000001 OR "PINK_FLOYD"."ID" IS NULL)
4 - access("STATUS"='OPEN')
6 - filter("STATUS"='OPEN')

Statistics
----------------------------------------------------------

0  recursive calls

0  db block gets

16341  consistent gets

8204  physical reads

0  redo size

2787  bytes sent via SQL*Net to client

609  bytes received via SQL*Net from client

8  SQL*Net roundtrips to/from client

0  sorts (memory)

0  sorts (disk)

100  rows processed

We notice the index is still used to efficiently find those rows of interest from the last partition, but a Full Table (Partition) Scan is performed to search for data from the other two partitions, for which with INDEXING OFF means the index does not contain entries that reference these partitions. As a result, this query is now much more expensive than it was previously as the index can not be used to exclusively find the rows of interest. The CBO within the one execution plan uses the index where it can and a full scan of the other partitions where it can’t use the index.

If however we were a little cleverer in how we designed our table and also created table subpartitions based on the STATUS column so that rows of interest resided in their own subpartitions and only set INDEXING ON for these subpartitions:

SQL> create table pink_floyd (id number, status varchar2(6), name varchar2(30))
indexing off
partition by range (id) subpartition by list(status)
subpartition template
(subpartition closed values ('CLOSED') indexing off, subpartition open values ('OPEN') indexing on)
(partition pf1 values less than (1000001),
partition pf2 values less than (2000001),
partition pf3 values less than (maxvalue))
enable row movement;

Table created.

Notice how only the subpartitions with a STATUS of ‘OPEN’ are now to be indexed. If we populate the table with the exact same data as before, we find the table and partial index have the follow characteristics:

SQL> select subpartition_position, subpartition_name, num_rows, indexing from dba_tab_subpartitions where table_name = 'PINK_FLOYD';

SUBPARTITION_POSITION SUBPARTITION_NAME      NUM_ROWS IND
--------------------- -------------------- ---------- ---
                    1 PF1_CLOSED              1000000 OFF
                    2 PF1_OPEN                      0 ON
                    1 PF2_CLOSED              1000000 OFF
                    2 PF2_OPEN                      0 ON
                    1 PF3_CLOSED               999900 OFF
                    2 PF3_OPEN                    100 ON

SQL> select index_name, num_rows, leaf_blocks, indexing from dba_indexes where index_name = 'PINK_FLOYD_STATUS_I';

INDEX_NAME             NUM_ROWS LEAF_BLOCKS INDEXIN
-------------------- ---------- ----------- -------
PINK_FLOYD_STATUS_I         100           1 PARTIAL

We can see that only the subpartitions with data of interest are now indexed. The resultant Partial global non-partitioned index is now tiny, with just the 100 index entries of interest residing in a single leaf block.

Just as importantly, a query searching for this data across the whole table is now extremely efficient and can be fully serviced by this tiny Partial index:

SQL> select * from pink_floyd where status = 'OPEN';

100 rows selected.

Execution Plan
----------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                                  | Name                | Rows | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
----------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                           |                     |   100 |  2500 |     2   (0)| 00:00:01 |       |       |
|   1 |  TABLE ACCESS BY GLOBAL INDEX ROWID BATCHED| PINK_FLOYD          |   100 |  2500 |     2   (0)| 00:00:01 | ROWID | ROWID |
|*  2 |   INDEX RANGE SCAN                         | PINK_FLOYD_STATUS_I |   100 |       |     1   (0)| 00:00:01 |       |       |
----------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - access("STATUS"='OPEN')

Statistics
----------------------------------------------------------

0  recursive calls

0  db block gets

16  consistent gets

0  physical reads

0  redo size

4479  bytes sent via SQL*Net to client

609  bytes received via SQL*Net from client

8  SQL*Net roundtrips to/from client

0  sorts (memory)

0  sorts (disk)

100  rows processed

The query can now be fully serviced by the Partial index as all subpartitions that could contain data of interest are indexed and because the data of interest is all neatly clustered within the table subpartitions, can be retrieved with far fewer consistent gets than previously.

If we ever wanted to access those STATUS values of ‘CLOSED’, the CBO can only do so via a Full Table Scan as such values are not indexed. However, as these values represent the vast majority of rows in the table, the Full Table Scan would be the most appropriate and efficient manner to access these rows any-ways.

The new Partial Index capabilities introduced in Oracle 12c enables us to easily use global (and local) indexes to just index data of interest without having to change the application. Such a capability has many potential uses.

More on Partial Indexes to come soon in Part II.

12c: Intro To Multiple Indexes On Same Column List (Repetition) July 2, 2013

Posted by Richard Foote in 12c, Multiple Indexes, Oracle Indexes.
6 comments

From an indexing perspective, one of the bigger ticket items introduced with Oracle Database 12c is the new capability to create multiple indexes on the same column list. It’s even a feature listed in the New Features Guide :) This can be particularly useful when you want to change the index definition in some way but you want to reduce the time in which the corresponding columns might not indexed.

Previous to 12c, you could not create an index if the same column list is already indexed and would generate an ORA-01408: such column list already indexed error. So, if you wanted to change an index from being say a B-Tree index to a Bitmap index, or from being Unique to Non-Unique or from being Non-Partitioned to Partitioned in same manner, etc. then you had to first drop the index and re-create it again as required. This means for the period in which the index is being re-created (which could be a considerable period for a larger index), the column list is not covered by an index, which might prove to be problematic.

Here, we create a partitioned table and create a Non-Unique, Non-Partitioned index on the ID column:

SQL> create table ziggy (id number, name varchar2(30)) partition by range (id) (partition ziggy1 values less than (1000), partition ziggy2 values less than (2000), partition ziggy3 values less than (maxvalue));

Table created.

SQL> insert into ziggy select rownum, 'DAVID BOWIE' from dual connect by level <=5000;

5000 rows created.

SQL> commit;

Commit complete.

SQL> create index ziggy_id_i1 on ziggy(id);              

Index created.

SQL> alter table ziggy add constraint ziggy_pk primary key(id);

Table altered.

If we decide for whatever reason we want to have say a Unique index instead:

SQL> create unique index ziggy_id_i2 on ziggy(id);
create unique index ziggy_id_i2 on ziggy(id)
                                         *
ERROR at line 1:
ORA-01408: such column list already indexed

No good, we can’t, even if we make it initially INVISIBLE:

SQL> create unique index ziggy_id_i2 on ziggy(id) invisible;
create unique index ziggy_id_i2 on ziggy(id) invisible
                                         *
ERROR at line 1:
ORA-01408: such column list already indexed

Because the index is used to police a PK constraint, we can’t even just drop the index:

SQL> drop index ziggy_id_i1;
drop index ziggy_id_i1
           *
ERROR at line 1:
ORA-02429: cannot drop index used for enforcement of unique/primary key

We have to first drop or disable the PK constraint, then drop the index, then re-create the index.

SQL> alter table ziggy drop primary key;

Table altered.

SQL> drop index ziggy_id_i1;

Index dropped.

SQL> alter table ziggy add primary key(id) using index (create unique index ziggy_id_i2 on ziggy(id));

Table altered.

This means the constraint is not automatically enforced (unless we disable it with validate, thus locking the table) and the ID column is not available via an index during the entire duration of creating the new index.

The 12c database has given us more flexibility in this regard.

Providing we create the index as INVISIBLE (meaning there is only ever the one Visible index on the column list), we can now create multiple indexes on the same column list, providing the new index has a different characteristic to existing indexes. So with a 12c database:

SQL> create unique index ziggy_id_i2 on ziggy(id) invisible;

Index created.

We now have two indexes on the same column list (the ID column). An attempt however to create or alter an index such that two visible indexes have the same column list will fail:

SQL> alter index ziggy_id_i2 visible;
alter index ziggy_id_i2 visible
*
ERROR at line 1:
ORA-14147: There is an existing VISIBLE index defined on the same set of
columns.

This means we can now (say) replace the index policing the PK constraint quicker (or convert a B-Tree to a Bitmap index or convert a Non-Partitioned index to a Partitioned index, etc.) as we don’t now have to wait for the new index to be created first:

SQL> alter table ziggy drop primary key;

Table altered.

SQL> drop index ziggy_id_i1;

Index dropped.

SQL> alter index ziggy_id_i2 visible;

Index altered.

SQL> alter table ziggy add constraint ziggy_pk primary key(id);

Table altered.

We can create as many indexes as we like on the ID, providing only one index is ever visible at a time and providing all the indexes have some form of differing characteristic. For example, all these indexes could potentially all co-exist:

SQL> create index ziggy_id_i3 on ziggy(id) local invisible;

Index created.

SQL> create bitmap index ziggy_id_i4 on ziggy(id) local invisible;

Index created.

SQL> create index ziggy_id_i5 on ziggy(id) reverse invisible;

Index created.

Of course, having many multiple indexes on the same column list in this manner is very likely a very bad idea …

Oracle Database 12c Released – Lots of Indexing Stuff To Talk About June 26, 2013

Posted by Richard Foote in 12c, Oracle Indexes.
10 comments

Finally, Oracle have released the long awaited 12c Database. You can now download it from OTN and other usual places. This is good news as my tongue was getting rather sore from having to bite on it for so long :)

From an indexing point of view, there are lots of fantastic new features, especially from a manageability perspective. The ability to create multiple indexes with the same column list, to partially index tables, with enhanced capabilities regarding online operations, with enhanced index monitoring capabilities, etc. etc. there’s plenty that I can now discuss here.

Unfortunately, all my presentations have been rejected for Oracle Openworld this year so I won’t be able to present them over there :(

Fortunately I’ll be able to discuss them all here in lots of detail in the coming weeks, so stay tuned :)

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