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https://issues.apache.org/jira/browse/DERBY-6940?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16058566#comment-16058566
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Bryan Pendleton commented on DERBY-6940:
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Regarding your observation about average row size and cost estimation, I
thought it would be worthwhile to note that there is some evidence that cost
estimation problems result in substantial impact to the Derby optimizer's query
planning algorithms. For example, see: DERBY-1905, DERBY-1260, DERBY-1205,
DERBY-1259, and DERBY-1007.
I'm not suggesting that we should take action on any of those issues right
away, just noting that it's important to keep cost estimation behaviors in mind
as we study the query optimizer's behaviors.
> 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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