The Ultimate Guide to High Availability with MariaDB

EXPLAIN ANALYZE

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This page contains preliminary documentation for the EXPLAIN ANALYZE feature.

This page describes a feature that's under development. The feature has not been released (even in beta), its interface and function may change, etc.

What it is

EXPLAIN ANALYZE SELECT works like in PostreSQL and other databases. It runs the SELECT, and then prints EXPLAIN output, where #record estimates are accompanied by the actual numbers encountered during the execution.

MariaDB [test]> explain analyze select * from tbl where key1 between 5000 and 5050 and col1='val-1'\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: tbl
         type: range
possible_keys: key1
          key: key1
      key_len: 5
          ref: NULL
        loops: 1
         rows: 51
       r_rows: 51
     filtered: 100.00
   r_filtered: 33.33
        Extra: Using index condition; Using where
1 row in set (0.00 sec)

Note the additional columns:

  • loops - Tells how many times table access was invoked. For non-join queries, this is always 1.
  • r_rows - The "real" counterpart of rows column. It shows how many records were returned by the access method (in the above example, by the range access).
  • r_filtered - The "real" counterpart of filtered column. It shows the fraction of records that was left after "Using where" condition was applied. (In the above example, the optimizer did not have any estimate for it, and pessimistically assumed that no records will be filtered out (filtered=100) while in actual execution 66% of the records were filtered, leaving 33%)

Where to get it

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