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https://issues.apache.org/jira/browse/DERBY-6940?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16061215#comment-16061215
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Harshvardhan Gupta commented on DERBY-6940:
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Also, I am creating another issue which will track the progress of integration
of the new statistics into selectivity estimates. Meanwhile, I would like to
invite your thoughts on other statistics we should consider. Most common values
and distribution buckets are on the top of my mind right now, I'd appreciate
your thoughts on the same. The average size of row written to disk is important
for cost estimation and a quick examination of derby optimizer's code reveals
that derby is already using it in some form, nevertheless I would revisit it
during my analysis of cost estimation over the course of this project.
> 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,
> EOFException_derby.log, EOFException.txt
>
>
> 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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