On Wednesday, April 2, 2014 4:21:44 PM UTC+1, Marcin Mirończuk wrote: > > Hi, > I have a: > - table opinion > - index CREATE INDEX opinionClassNameIndex ON OPINION(CLASSNAME ) > > This query is very fast and use index: > select count(*) from opinion where className like 'opinion%'; > 307 ms > > SELECT > COUNT(*) > FROM PUBLIC.OPINION > /* PUBLIC.OPINIONCLASSNAMEINDEX: CLASSNAME >= 'opinion' > AND CLASSNAME < 'opinioo' > */ > /* scanCount: 944400 */ > WHERE CLASSNAME LIKE 'opinion%' > /* > OPINION.OPINIONCLASSNAMEINDEX read: 18481 > */ > > This query is very slow and not use index: > select count(*) from opinion where className not like 'opinion%'; > 4826 ms > > SELECT > COUNT(*) > FROM PUBLIC.OPINION > /* PUBLIC.OPINION.tableScan */ > /* scanCount: 1099380 */ > WHERE NOT (CLASSNAME LIKE 'opinion%') > /* > total: 545475 > OPINION.OPINION_DATA read: 545470 (99%) > overflow read: 5 (0%) > */ > > Why NOT-LIKE is extremely slow? How I can improve it? > > Best > > > Marcin M. >
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