Richard Foote's Oracle Blog

Larger Block Tablespace For Indexes Revisted: Part II (Money) February 23, 2009

In Part I I looked at Robin Schumacher’s “classic” example of the “so-called” benefits of rebuilding an index into a larger block tablespace. I started by highlighting that by simply rebuilding an index in a larger block tablespace and (say) halving the number of associated index blocks, it doesn’t necessarily mean the index will result in a “flatter structure” , that the index height will reduce. Robin’s demo is in fact a perfect example of this.

Let me start Part II by following this up with a little story …

“I went with a friend of mine to get some cash from the bank the other day in order to buy the latest David Bowie Box-Set. I got my cash in $50 notes but being an efficient, cost saving sort of bloke, my mate got out the same amount of cash in $100 notes. Although we both had the same amount of cash, his wallet was that little bit more compact and “efficient” than mine as he had less actual bank notes to carry.

However, when we got to the record store, we were surprised to discover that the actual “cost” of the David Bowie Box-Set was 2 bank notes of any domination, but with no change being given. Therefore, it cost me 2 x $50 notes to make my purchase. Unfortunately for my mate, it cost him 2 x $100 for the same thing as $100 notes were the smallest denomination he could use. Yes, I was a little bit mean not lending him some of my $50 notes but I was a little disappointed myself for not having any $5 notes on me at the time 😉 ”

OK, it’s not a perfect analogy but you perhaps get the point …

If you have to pay with a quantity of bank notes and you only have larger bank notes, you end up paying more than you would if you could only have paid with the same number of smaller denomination bank notes.

If you have to pay for an index scan in database blocks and you’re forced to use larger index blocks, you actually end up paying more if you have to read the same number of index blocks anyways.

This is one of the potential dangers with rebuilding indexes in a larger block tablespace. And like I said, Robin’s little demo is a perfect example of this. You might indeed reduce and halve the number of index blocks but this might not be sufficient to actually flatten the index structure and reduce the actual height of the index. The height of the index after rebuilding the index can remain the same, so the minimum cost of performing a small range scan increases. Even if you reduce the index height, you can still end up paying more if the savings do not compensate you enough for the additional overhead associated with now having to read and process larger index blocks. 

Therefore, you potentially start paying more for smaller index range scans because you might not actually reduce the number of index blocks you visit for these types of index scans.

Taking exactly the same table/index definitions and data used to replicate Robin’s example in Part I, let’s see if there’s any difference in the costs associated with performing a number of small index range scans after rebuilding the index in a 16K block tablespace.

Again, it’s a simple case of just giving it a go and see for yourself. What resources are used if you perform a series of small index scans ? Do things really run faster ? Do we really use less resources ?  Having rebuilt such an index in a larger block tablespace and halved the number of associated leaf blocks, are we really better off ?

Let’s begin by setting up the same example as Robin’s demo as before …

SQL> create table bowie (id number not null, value varchar2(10));

Table created.

SQL> insert into bowie select rownum, ‘BOWIE’ from dual connect by level <=187200;

187200 rows created.

SQL> commit;

Commit complete.

SQL> create index bowie_idx on bowie(id);

Index created.

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>’BOWIE’, cascade=>true, estimate_percent=>null, method_opt=> ‘FOR ALL COLUMNS SIZE 1’);

PL/SQL procedure successfully completed.

SQL> select index_name, blevel from user_indexes where index_name = ‘BOWIE_IDX’;

INDEX_NAME BLEVEL
---------- ------
BOWIE_IDX       1

SQL> analyze index bowie_idx validate structure;

Index analyzed.

SQL> select height, btree_space, used_space from index_stats;

HEIGHT BTREE_SPACE USED_SPACE
------ ----------- ----------
     2     3336032    2988168

Note the index height when built in an 8K block tablespace is 2 …

Let’s now capture the current amount of CPU used by the session:

SQL> select n.name, s.value from v$sesstat s, v$statname n where s.statistic# = n.statistic# and s.sid = 134 and (n.name = ‘CPU used by this session’ or n.name = ‘consistent gets’ or n.name = ‘physical reads’);

NAME                          VALUE
------------------------ ----------
CPU used by this session      36555
consistent gets            22520369
physical reads                 3750

Now we run a series of small index range scans …

SQL> set timing on

SQL> declare
  2  v_id    number;
  3  v_value varchar2(10);
  4  begin
  5   for o in 1..10 loop
  6    for i in 1..187200 loop
  7    select id, value into v_id, v_value from bowie where id = i;
  8    end loop;
  9  end loop;
 10  end;
 11  /

PL/SQL procedure successfully completed.

Elapsed: 00:01:28.42

Let’s see how our resource stats have changed …

SQL> select n.name, s.value from v$sesstat s, v$statname n where s.statistic# = n.statistic# and s.sid = 134 and (n.name = ‘CPU used by this session’ or n.name = ‘consistent gets’ or n.name = ‘physical reads’);

NAME                          VALUE
------------------------ ----------
CPU used by this session      45346
consistent gets            28140519
physical reads                 3750

We note we have used approximately 87.91 CPU seconds. (Note: You can run this a number of times and determine an average figure).

Let’s now rebuild the index again in a 16K block tablespace:

SQL> alter index bowie_idx rebuild tablespace ts_16k;

Index altered.

SQL> select index_name, blevel from user_indexes where index_name = ‘BOWIE_IDX’;

INDEX_NAME BLEVEL
---------- ------
BOWIE_IDX       1

SQL> analyze index bowie_idx validate structure;

Index analyzed.

SQL> select height, btree_space, used_space from index_stats;

    HEIGHT BTREE_SPACE USED_SPACE
---------- ----------- ----------
         2     3351776    2985662 

Note the index height remains at 2 …

If we run the same series of small index scans:

SQL> select n.name, s.value from v$sesstat s, v$statname n where s.statistic# = n.statistic# and s.sid = 134 and (n.name = ‘CPU used by this session’ or n.name = ‘consistent gets’ or n.name = ‘physical reads’);

NAME                          VALUE
------------------------ ----------
CPU used by this session      45381
consistent gets            28142640
physical reads                 3957

SQL> declare
  2  v_id    number;
  3  v_value varchar2(10);
  4  begin
  5   for o in 1..10 loop
  6    for i in 1..187200 loop
  7    select id, value into v_id, v_value from bowie where id = i;
  8    end loop;
  9  end loop;
 10  end;
 11  /

PL/SQL procedure successfully completed.

Elapsed: 00:01:42.44

SQL> select n.name, s.value from v$sesstat s, v$statname n where s.statistic# = n.statistic# and s.sid = 134 and (n.name = ‘CPU used by this session’ or n.name = ‘consistent gets’ or n.name = ‘physical reads’);

NAME                          VALUE
------------------------ ----------
CPU used by this session      54484
consistent gets            33760690
physical reads                 3957

We note that elapsed times have increased and we have now increased our overall CPU consumption to 91.03 CPU seconds as well.

As we can see, there has been no advantage with rebuilding the in index in the 16K block tablespace for these smaller index scans. In fact, there’s actually been an increase in the overall elapsed times and an increase in the overall CPU. Performance has not improved but has in fact worsened overall for these queries after rebuilding the index in a larger block tablespace.

You begin to get the point …

And of course, indexes are generally far more typically to be used in small index range scan operations than they are in performing Index Fast Full Index Scans. Just compare the numbers of index fast full scans vs. index fetch by key operations in your databases if you want some indication. 

However, Robin’s specific example used a SQL statement that performed an Index Fast Full Scan. Surely, such operations would improve dramatically if indexes were only rebuilt in a larger block tablespace ? Surely such operations would be able to read data more quickly, especially if we have to go to disk, as we would be able to read more data with each associated multiblock read if the index were built in a larger block tablespace ? Surely it would be worth all the extra effort and management considerations ?

Things will run at least 2 times faster, maybe even 150 times faster, right  😉

We’ll see how the results can be a tad “disappointing” in the next post …