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From "Martin Hajduch (JIRA)" <j...@apache.org>
Subject [jira] Created: (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 Wed, 05 Nov 2008 17:01:46 GMT
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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