beer planet » explain

MySQL Conference Liveblogging: EXPLAIN Demystified (Tuesday 2:00PM)

Artem Russakovskii — Tue, 15 Apr 2008 21:27:03 +0000

Baron Schwartz presents
only works for SELECTs
nobody dares admit if they've never seen EXPLAIN
MySQL actually executes the query
at each JOIN, instead of executing the query, it fills the EXPLAIN result set
everything is a JOIN (even SELECT 1)
Columns in EXPLAIN

id: which SELECT the row belongs to
select_type

simple
subquery
derived
union
union result

table: the table accessed or its alias
type:

join
range
…

possible_keys: which indexes looked useful to the optimizer
key: which index(es) the optimizer chose
key_len: the number of bytes of the index MySQL will use
ref: which columns/constants from preceding tables are used for lookups in the index named in the key column
rows: estimated number of rows to read
extra

using index: covering index
using where: server post-filters rows from storage engine
using temporary: an implicit temp table (for sorting or grouping rows, DISTINCT). No indication of whether the temp table is in memory or on disk
using filesort: external sort to order result. No indication of which algorithm MySQL will use

shows an insane EXPLAIN output with 8 EXPLAIN rows
maatkit includes a tool called mk-visual-explain, which can construct a formatted tree
Baron shows a demo and answers questions
EXPLAIN EXTENDED followed by SHOW WARNINGS will give more output about how a query is executed

Similar Posts: