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The locution "EXISTS(SELECT ... LIMIT 1)" seems to be rather common among people who don't realize that the database already performs optimizations equivalent to putting LIMIT 1 in the sub-select. Unfortunately, this was actually making things worse, because it prevented us from optimizing such EXISTS clauses into semi or anti joins. Teach simplify_EXISTS_query() to suppress constant-positive LIMIT clauses. That fixes the semi/anti-join case, and may help marginally even for cases that have to be left as sub-SELECTs. Marti Raudsepp, reviewed by David Rowley |
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