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Harshvardhan Gupta commented on DERBY-6940: ------------------------------------------- While debugging the failed test cases I seem to have reproduced a previously know issue. Please take a look at my comment on DERBY-3219. > Enhance derby statistics for more accurate selectivity estimates. > ----------------------------------------------------------------- > > Key: DERBY-6940 > URL: https://issues.apache.org/jira/browse/DERBY-6940 > Project: Derby > Issue Type: Sub-task > Components: SQL > Reporter: Harshvardhan Gupta > Assignee: Harshvardhan Gupta > Priority: Minor > Attachments: DERBY-6940_2.diff, DERBY-6940_3.diff, derby-6940.diff > > > Derby should collect extra statistics during index build time, statistics > refresh time which will help optimizer make more precise selectivity > estimates and chose better execution paths. > We eventually want to utilize the new statistics to make better selectivity > estimates / cost estimates that will help find the best query plan. Currently > Derby keeps two type of stats - the total row count and the number of unique > values. > We are initially extending the stats to include null count, the minimum value > and maximum value associated with each of the columns of an index. This would > be useful in selectivity estimates for operators such as [ IS NULL, <, <=, >, > >= ] , all of which currently rely on hardwired selectivity estimates. -- This message was sent by Atlassian JIRA (v6.4.14#64029)