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

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CHAPTER 11 ■ INDEXES
Myth: Space Is Never Reused in an Index
This is a myth that I would like to dispel once and for all: space is reused in an index. The myth
goes like this: you have a table, T, in which there is a column, X. At some point, you put the
value X=5in the table. Later you delete it. The myth is that the space used by X=5will not be
reused unless you put X=5back into the index later. The myth states that once an index slot is
used, it will be there forever and can be reused only by the same value. A corollary to this is the
myth that free space is never returned to the index structure, and a block will never be reused.
Again, this is simply not true.
The first part of the myth is trivial to disprove. All we need to do is to create a table
like this:
ops$tkyte@ORA10GR1> create table t ( x int, constraint t_pk primary key(x) );
Table created.
ops$tkyte@ORA10GR1> insert into t values (1);
1 row created.
ops$tkyte@ORA10GR1> insert into t values (2);
1 row created.
ops$tkyte@ORA10GR1> insert into t values (9999999999);
1 row created.
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
So, according to the myth, if I delete from T where X=2, that space will never be reused
unless I reinsert the number 2. Currently, this index is using one leaf block of space. If the
index key entries are never reused upon deletion, and I keep inserting and deleting and never
reuse a value, this index should grow like crazy. Let’s see:
ops$tkyte@ORA10GR1> begin
2 for i in 2 .. 999999
3 loop
4 delete from t where x = i;
5 commit;
6 insert into t values (i+1);
7 commit;
8 end loop;
9 end;
10 /