LIKE operator optimizations and concatenation
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Key: DERBY-4791
URL: https://issues.apache.org/jira/browse/DERBY-4791
Project: Derby
Issue Type: Improvement
Components: SQL
Affects Versions: 10.6.1.0
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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