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From "Uriah Eisenstein (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-4791) LIKE operator optimizations and concatenation
Date Sat, 04 Sep 2010 16:37:33 GMT
LIKE operator optimizations and concatenation

                 Key: DERBY-4791
                 URL: https://issues.apache.org/jira/browse/DERBY-4791
             Project: Derby
          Issue Type: Improvement
          Components: SQL
    Affects Versions:
         Environment: All
            Reporter: Uriah Eisenstein

While queries of the form "<column> LIKE ' <string-constant>%' " are optimized
into index scans, the equivalent expression using a concatenation "<column> LIKE ' <string-constant>'
|| '%' " would result in a table scan. 
Queries of the form "<column> LIKE ?" are optimizable using an internally generated
parameter, so it doesn't seem far-fetched to generate such a parameter for a concatenation
of strings (or other string expressions) as well, once its value has been calculated. This
is of course limited to cases where the result of the string expression can be calculated
once, i.e. it is independent of columns in the query.
It is sometimes possible to work around this by manually adding the " x >= '<string-constant>'
AND x < '<string-constant>\uffff...'  " condition.

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