I am trying to optimize a MySQL select request:
SELECT * FROM `sales`
WHERE ((sales.private = false AND (sales.buyer_id IS NULL OR NOT sales.buyer_id=142)
AND (sales.merchand_id IS NULL OR NOT sales.merchand_id=142)
AND (sales.private_item = false) )
AND ((sales.buyer_id=32 OR sales.merchand_id=32)
AND (sales.admin=0 AND NOT sales.type IN ('book'))))
ORDER BY sales.created_at DESC, sales.id DESC LIMIT 0, 10;
The schema of the table is
mysql> SHOW columns from sales;
+------------------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------------------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| type | varchar(255) | YES | MUL | NULL | |
| buyer_id | int(11) | YES | MUL | NULL | |
| merchand_id | int(11) | YES | MUL | NULL | |
| private | tinyint(1) | YES | | 0 | |
| admin | tinyint(1) | YES | | 0 | |
| created_at | datetime | YES | | NULL | |
| updated_at | datetime | YES | | NULL | |
| country_id | int(11) | YES | MUL | 0 | |
| private_item | tinyint(1) | YES | | 0 | |
+------------------------+--------------+------+-----+---------+----------------+
The indexes are:
mysql> show indexes from sales;
+-----------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-----------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| sales | 0 | PRIMARY | 1 | id | A | 286509 | NULL | NULL | | BTREE | |
| sales | 1 | index_sales_on_type | 1 | type | A | 123 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_on_buyer_id | 1 | buyer_id | A | 40929 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_on_merchand_id | 1 | merchand_id | A | 40929 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_on_country_id | 1 | country_id | A | 6 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_on_type_and_country_id | 1 | type | A | 151 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_on_type_and_country_id | 2 | country_id | A | 428 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 1 | buyer_id | A | 35813 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 2 | merchand_id | A | 286509 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 3 | private_item| A | 285009 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 4 | admin | A | 285009 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 5 | type | A | 285009 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 6 | private | A | 285009 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 7 | created_at | A | 285009 | NULL | NULL | YES | BTREE | |
+-------+------------+------------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
When doing the query it uses the index_sales_on_type_and_country_id even if there is no country_id in the query...
The query takes 2.5 seconds with this index.
But when I use USE INDEX(index_sales_viewed) it goes down to 0.2 seconds.
Here is the EXPLAIN of the query:
+----+-------------+-----------------+------+----------------------------------------------+------+---------+------+--------+---------------------
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------------+------+----------------------------------------------+------+---------+------+--------+---------------------
| 1 | SIMPLE | sales | range | see bellow | index_sales_on_type_and_country_id | 258 | NULL | 208725 | Using where; Using filesort |
+----+-------------+-----------------+------+----------------------------------------------+------+---------+------+--------+---------------------
the possible keys are :
index_sales_on_type,
index_sales_on_buyer_id,
index_sales_on_merchand_id,
index_sales_on_type_and_country_id,
index_sales_public_recent_activity
Why doesn't MySQL use index_sales_viewed by default? Could there be a better index?
Thank you!
