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Oracle LIKE clause searches with text indexes

Oracle Tips by Burleson Consulting


One serious SQL performance problem occurs when you use the SQL 'LIKE clause' operator to find a string within a large Oracle table column (e.g. VARCHAR(2000), CLOB, BLOB):


Select stuff from bigtab where text_column like '%ipod%';
Select stuff from bigtab where full_name like '%JONES';


Because standard Oracle cannot index into a large column, there 'like' queries cause full-table scans, and Oracle must examine every row in the table, even when the result set is very small.  These unnecessary full-table scans are a problem:


  1. Large-table full-table scans increase the load on the disk I/O sub-system

  2. Small table full table scans (in the data buffer) cause high consistent gets and drive-up CPU consumption

For details, see Oracle SQL "like clause" and index access.


And see here, how to create a con*text Oracle text index.

If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.



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