diff options
Diffstat (limited to 'railties/guides/source/active_record_querying.textile')
| -rw-r--r-- | railties/guides/source/active_record_querying.textile | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/railties/guides/source/active_record_querying.textile b/railties/guides/source/active_record_querying.textile index ad12dca7e8..c4724f182e 100644 --- a/railties/guides/source/active_record_querying.textile +++ b/railties/guides/source/active_record_querying.textile @@ -1287,6 +1287,7 @@ User.where(:id => 1).joins(:posts).explain may yield <plain> +EXPLAIN for: SELECT `users`.* FROM `users` INNER JOIN `posts` ON `posts`.`user_id` = `users`.`id` WHERE `users`.`id` = 1 <plus>----<plus>-------------<plus>-------<plus>-------<plus>---------------<plus>---------<plus>---------<plus>-------<plus>------<plus>-------------<plus> | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | <plus>----<plus>-------------<plus>-------<plus>-------<plus>---------------<plus>---------<plus>---------<plus>-------<plus>------<plus>-------------<plus> @@ -1302,6 +1303,7 @@ Active Record performs a pretty printing that emulates the one of the database shells. So, the same query running with the PostgreSQL adapter would yield instead <plain> +EXPLAIN for: SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id" WHERE "users"."id" = 1 QUERY PLAN ------------------------------------------------------------------------------ Nested Loop Left Join (cost=0.00..37.24 rows=8 width=0) @@ -1324,12 +1326,15 @@ User.where(:id => 1).includes(:posts).explain yields <plain> +EXPLAIN for: SELECT `users`.* FROM `users` WHERE `users`.`id` = 1 <plus>----<plus>-------------<plus>-------<plus>-------<plus>---------------<plus>---------<plus>---------<plus>-------<plus>------<plus>-------<plus> | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | <plus>----<plus>-------------<plus>-------<plus>-------<plus>---------------<plus>---------<plus>---------<plus>-------<plus>------<plus>-------<plus> | 1 | SIMPLE | users | const | PRIMARY | PRIMARY | 4 | const | 1 | | <plus>----<plus>-------------<plus>-------<plus>-------<plus>---------------<plus>---------<plus>---------<plus>-------<plus>------<plus>-------<plus> 1 row in set (0.00 sec) + +EXPLAIN for: SELECT `posts`.* FROM `posts` WHERE `posts`.`user_id` IN (1) <plus>----<plus>-------------<plus>-------<plus>------<plus>---------------<plus>------<plus>---------<plus>------<plus>------<plus>-------------<plus> | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | <plus>----<plus>-------------<plus>-------<plus>------<plus>---------------<plus>------<plus>---------<plus>------<plus>------<plus>-------------<plus> @@ -1339,3 +1344,14 @@ yields </plain> under MySQL. + +h4. Interpreting EXPLAIN + +Interpretation of the output of EXPLAIN is beyond the scope of this guide. The +following pointers may be helpful: + +* SQLite3: "EXPLAIN QUERY PLAN":http://www.sqlite.org/eqp.html + +* MySQL: "EXPLAIN Output Format":http://dev.mysql.com/doc/refman/5.6/en/explain-output.html + +* PostgreSQL: "Using EXPLAIN":http://www.postgresql.org/docs/current/static/using-explain.html |