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 ?
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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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