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https://issues.apache.org/jira/browse/DERBY-3937?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12659582#action_12659582
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Myrna van Lunteren commented on DERBY-3937:
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To work around this limitation, apart from what's already replied, and you may
have considered this already...but if the exact precise count is not essential
(and select count(*) is more habitual), and you don't have a lot of deletes,
maybe an id/(generated)sequence # column - where one'd go with the highest
sequence number to represent the count - would work in the application?
Or, if the exact count is essential and needs to be accessible at a snap...not
according to database normalization rules maybe, and definitely a 'crutch', but
who knows, it might work for your application: a table that only has a counter,
which gets updated for every insert & delete?
> 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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