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From "Harshvardhan Gupta (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-6940) Enhance derby statistics for more accurate selectivity estimates.
Date Thu, 22 Jun 2017 22:08:00 GMT

    [ https://issues.apache.org/jira/browse/DERBY-6940?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16060091#comment-16060091
] 

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.



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