Improve efficiency of pagination count query #2525
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When paginating results, a COUNT is performed to get the total number of results that would be returned by the query. For queries with a DISTINCT in their SELECT, this requires the DB to perform a full sort over the entire SELECT table.* columns (sometimes 15+) which can be very slow. Since this is only being used for a COUNT, only a unique identifier is needed to do the count which is already unique so does not require sorting. This halves the EXPLAIN cost of some of the service_plans queries since that is currently sorting over 15+ fields just to return a single COUNT. Co-Authored-By: Philipp Thun <philipp.thun@sap.com>
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This doesn't actually perform correctly if the |
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A short explanation of the proposed change:
Only count the primary key fields for DISTINCT queries
An explanation of the use cases your change solves
When paginating results, a COUNT is performed to get the total number of
results that would be returned by the query. For queries with a
DISTINCT in their SELECT, this requires the DB to perform a full
sort over the entire SELECT table.* columns (sometimes 15+) which can be
very slow. Since this is only being used for a COUNT, only a unique
identifier is needed to do the count which is already unique so does not
require sorting. This halves the EXPLAIN cost of some of the
service_plans queries since that is currently sorting over 15+ fields
just to return a single COUNT.
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I have made this pull request to the
mainbranchI have run all the unit tests using
bundle exec rakeI have run CF Acceptance Tests