.. _prefix_index_queries_optimization:

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Prefix Index Queries Optimization
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|Percona Server| has ported Prefix Index Queries Optimization feature from
Facebook patch for |MySQL|.

Prior to this |InnoDB| would always fetch the clustered index for all prefix
columns in an index, even when the value of a particular record was smaller
than the prefix length. This implementation optimizes that case to use the
record from the secondary index and avoid the extra lookup.

Status Variables
================

.. variable:: Innodb_secondary_index_triggered_cluster_reads

     :version 5.7.18-14: Implemented
     :vartype: Numeric
     :scope: Global

This variable shows the number of times secondary index lookup triggered
cluster lookup.

.. variable:: Innodb_secondary_index_triggered_cluster_reads_avoided

     :version 5.7.18-14: Implemented
     :vartype: Numeric
     :scope: Global

This variable shows the number of times prefix optimization avoided
triggering cluster lookup.
