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https://issues.apache.org/jira/browse/DERBY-3937?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12737087#action_12737087
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martin edited comment on DERBY-3937 at 7/30/09 5:45 AM:
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> The current implementations of indexes and base tables do not maintain an
> exact count of rows.
Index could be useful even if it does not maintain "exact count". If scanning
index can provide the result, it should be faster than scanning the table
itself. Is it feasible?
was (Author: mstanik):
> The current implementations of indexes and base tables do not maintain an
exact count of rows.
Index could be useful even if it does not maintain "exact count". If scanning
index can provide the result, scanning index should be faster than scanning the
table itself. Is it feasible?
> 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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