When btree_space is less than used_space

Question

Hi all,
Can anybody let me know on what condition btree_space is less than used_space in Oracle 9.2.0.8
SQL> SELECT HEIGHT,BLOCKS,BTREE_SPACE, USED_SPACE, PCT_USED,DEL_LF_ROWS,BR_ROWS
FROM INDEX_STATS;
    HEIGHT     BLOCKS BTREE_SPACE USED_SPACE   PCT_USED DEL_LF_ROWS    BR_ROWS
         3       1024     5425220    6146870        114         571        928

Answer 1

user13389425 wrote:
Can anybody let me know on what condition btree_space is less than used_space in Oracle 9.2.0.8
SQL> SELECT HEIGHT,BLOCKS,BTREE_SPACE, USED_SPACE, PCT_USED,DEL_LF_ROWS,BR_ROWS
FROM INDEX_STATS;
HEIGHT     BLOCKS BTREE_SPACE USED_SPACE   PCT_USED DEL_LF_ROWS    BR_ROWS
3       1024     5425220    6146870        114         571        928
This may be the effect of a concurrency issue on the index that has resulted in a rapid growth in the number of ITL entries in some of your index leaf blocks.
The USED_SPACE is a measure of the actual size of the data in the index; the btree_space is Oracle's estimate of the available space in the index on the assumption that the size of the largest ITL is the size of every ITL in the index.
Your br_rows figure shows that you have about 930 leaf blocks, which as 8KB per leaf block and INITRANS of 10 should give you a bree_space of about 7M, but your btree space is only about 5.2 MB, which suggests that Oracle has seen some ITLs with about 80 entries.
Oracle has a defect in its leaf node split algorithm that can make the ITL to grow much bigger than it needs to be. Because you are running 9.2 you can work around this problem by setting maxtrans on the index to a reasonable value - since you've decided to set initrans to 10, then setting maxtrans to a slightly larger value (and living with a few TX mode 4 waits) is probably sensible. Unfortunately 10g ignores maxtrans.
There's a series of articles about this on my blog - the first one is at http://jonathanlewis.wordpress.com/2009/07/20/index-quiz-1/ and the catalogue to the series is at http://jonathanlewis.wordpress.com/2009/09/19/index-itls/
Regards
Jonathan Lewis