Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

CHAPTER 11 ■ INDEXES 483
ops$tkyte@ORA10GR1> analyze index t_pk validate structure;
Index analyzed.
ops$tkyte@ORA10GR1> select lf_blks, br_blks, btree_space
2 from index_stats;
LF_BLKS BR_BLKS BTREE_SPACE
---------- ---------- -----------
1 0 7996
This shows the space in the index was reused. As with most myths, however, there is a
nugget of truth in there. The truth is that the space used by that initial number 2 (in between
1 and 9,999,999,999) would remain on that index block forever. The index will not “coalesce”
itself. This means if I load a table with values 1 to 500,000 and then delete every other row (all
of the even numbers), there will be 250,000 “holes” in the index on that column. Only if I reinsert
data that will fit onto a block where there is a hole will the space be reused. Oracle will
make no attempt to “shrink” or compact the index. This can be done via an ALTER INDEX
REBUILD or COALESCE command. On the other hand, if I load a table with values 1 to 500,000
and then delete from the table every row where the value was 250,000 or less, I would find the
blocks that were cleaned out of the index were put back onto the freelist for the index. This
space can be totally reused.
If you recall, this was the second myth: index space is never “reclaimed.” It states that
once an index block is used, it will be stuck in that place in the index structure forever and will
only be reused if you insert data that would go into that place in the index anyway. We can
show that this is false as well. First, we need to build a table with about 500,000 rows in it. For
that, we’ll use the big_table script. After we have that table with its corresponding primary key
index, we’ll measure how many leaf blocks are in the index and how many blocks are on the
freelist for the index. Remember, with an index, a block will only be on the freelist if the block
is entirely empty, unlike a table. So any blocks we see on the freelist are completely empty and
available for reuse:
ops$tkyte@ORA10GR1> select count(*) from big_table;
COUNT(*)
----------
500000
ops$tkyte@ORA10GR1> declare
2 l_freelist_blocks number;
3 begin
4 dbms_space.free_blocks
5 ( segment_owner => user,
6 segment_name => 'BIG_TABLE_PK',
7 segment_type => 'INDEX',
8 freelist_group_id => 0,
9 free_blks => l_freelist_blocks );
10 dbms_output.put_line( 'blocks on freelist = ' || l_freelist_blocks );
11 end;