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_small_table_threshold tips

Oracle Database Tips by Donald Burleson

7 June 2015

Question:  How does Oracle distinguish the size difference between a large table and a small table?  I see that the size of large table and small tables are important to the optimizer.

Answer:  In Oracle7 and Oracle8, the size of small tables was defined by the small_table_threshold parameter, but in Oracle8i and beyond, the definition of a small table was changed to be any tables whose total data blocks is greater than 2% of the number of blocks defined by db_cache_size.

You can still change the small table threshold with the now-hidden parameter _small_table_threshold).

The hidden parameter "_small_table_threshold" defines the number of blocks to consider a table as being "small".

You can see the value of the hidden parameter _small_table_threshold with the following query against the x$ fixed tables:

col name format A25
col value format A10
col description format A30
   x.ksppinm name,
   y.ksppstvl value,
   x.ksppdesc description
   x$ksppi x,
   x$ksppcv y
   x.inst_id = userenv('Instance')
   and y.inst_id = userenv('Instance')
   and x.indx = y.indx
   and x.ksppinm ='_small_table_threshold'
order by

NAME                      VALUE      DESCRIPTION
------------------------- ---------- ------------------------------
_small_table_threshold    368         threshold level of table size for direct reads

Any table having more than 5 times the number of blocks in "_small_table_threshold" (if you leave it at default value) will automatically use direct path reads for serial full table scans (FTS).

Because the definition of a small table scales with the size of the SGA, a 200 block table may be considered small on one database and large in another.

Very large table thresholds:

Conversely, here is a hidden parameter governing the size of a very large table.  See _very_large_object_threshold for details




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