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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] Updated: (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 Tue, 29 Jun 2010 22:38:51 GMT

     [ https://issues.apache.org/jira/browse/DERBY-3937?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Mike Matrigali updated DERBY-3937:
----------------------------------


I believe that optimizer does consider doing a scan of an index vs. base table for execution
of the count(*).  You should be able to tell from the query plan.  

> 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: Store
>         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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