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From "martin (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3937) Select count(*) scans all the rows (and is therefore slow with big tables), is the amount of rows not available/known for example in index ?
Date Thu, 05 Feb 2009 15:53:59 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3937?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12670803#action_12670803
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martin commented on DERBY-3937:
-------------------------------

if engine does not know number of rows in table, does not disable it query optimizer?
optimizer should work on big tables, not on small.

> Select count(*) scans all the rows (and is therefore slow with big tables), is the amount
of rows not available/known for example in index ?
> --------------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: DERBY-3937
>                 URL: https://issues.apache.org/jira/browse/DERBY-3937
>             Project: Derby
>          Issue Type: Improvement
>          Components: Performance
>         Environment: Any
>            Reporter: Martin Hajduch
>
> Create table with 5000000 rows. Create index on unique ID. Select count(*) on such table
is going to take quite some time.
> Shouldn't the index contain amount of indexed rows and the value taken from there ?
> Additionally, queries of the form select count(*) from table where col1=value; take lots
of time (depending on amount of rows satisfying WHERE clause) even if index on col1 exists.
Isn't it possible to find first and last occurence in the index, and then calculate amount
of rows more effectively then scanning through all of them ?

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