我正在尝试优化使用 MySQL 5.1 中的视图的查询。似乎即使我从视图中选择 1 列，它也总是会进行全表扫描。这是预期的行为吗？
对于我在下面的第一个查询中指定的表，该视图只是一个 SELECT "All Columns From These Tables - NOT *"。
这是我从构成视图的查询中选择索引列 PromotionID 时的解释输出。如您所见，它与视图上的输出有很大不同。
EXPLAIN SELECT pb.PromotionID FROM PromotionBase pb INNER JOIN PromotionCart pct ON pb.PromotionID = pct.PromotionID INNER JOIN PromotionCode pc ON pb.PromotionID = pc.PromotionID WHERE pc.PromotionCode = '5TAFF312C0NT'\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: pc
type: const
possible_keys: PRIMARY,fk_pc_pb
key: PRIMARY
key_len: 302
ref: const
rows: 1
Extra:
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: pb
type: const
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: const
rows: 1
Extra: Using index
*************************** 3. row ***************************
id: 1
select_type: SIMPLE
table: pct
type: const
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: const
rows: 1
Extra: Using index
3 rows in set (0.00 sec)
当我从视图中选择相同的东西时的输出
EXPLAIN SELECT vpc.PromotionID FROM vw_PromotionCode vpc WHERE vpc.PromotionCode = '5TAFF312C0NT'\G;
*************************** 1. row ***************************
id: 1
select_type: PRIMARY
table: <derived2>
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 5830
Extra: Using where
*************************** 2. row ***************************
id: 2
select_type: DERIVED
table: pcart
type: index
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: NULL
rows: 33
Extra: Using index
*************************** 3. row ***************************
id: 2
select_type: DERIVED
table: pb
type: eq_ref
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: readyinteractive.pcart.PromotionID
rows: 1
Extra:
*************************** 4. row ***************************
id: 2
select_type: DERIVED
table: pc
type: ref
possible_keys: fk_pc_pb
key: fk_pc_pb
key_len: 4
ref: readyinteractive.pb.PromotionID
rows: 249
Extra: Using where
*************************** 5. row ***************************
id: 3
select_type: UNION
table: pp
type: index
possible_keys: PRIMARY
key: pp_p
key_len: 4
ref: NULL
rows: 1
Extra: Using index
*************************** 6. row ***************************
id: 3
select_type: UNION
table: pb
type: eq_ref
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: readyinteractive.pp.PromotionID
rows: 1
Extra:
*************************** 7. row ***************************
id: 3
select_type: UNION
table: pc
type: ref
possible_keys: fk_pc_pb
key: fk_pc_pb
key_len: 4
ref: readyinteractive.pb.PromotionID
rows: 249
Extra: Using where
*************************** 8. row ***************************
id: 4
select_type: UNION
table: pcp
type: index
possible_keys: PRIMARY
key: pcp_cp
key_len: 4
ref: NULL
rows: 1
Extra: Using index
*************************** 9. row ***************************
id: 4
select_type: UNION
table: pb
type: eq_ref
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: readyinteractive.pcp.PromotionID
rows: 1
Extra:
*************************** 10. row ***************************
id: 4
select_type: UNION
table: pc
type: ref
possible_keys: fk_pc_pb
key: fk_pc_pb
key_len: 4
ref: readyinteractive.pb.PromotionID
rows: 249
Extra: Using where
*************************** 11. row ***************************
id: 5
select_type: UNION
table: ppc
type: index
possible_keys: PRIMARY
key: ppc_pc
key_len: 4
ref: NULL
rows: 1
Extra: Using index
*************************** 12. row ***************************
id: 5
select_type: UNION
table: pb
type: eq_ref
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: readyinteractive.ppc.PromotionID
rows: 1
Extra:
*************************** 13. row ***************************
id: 5
select_type: UNION
table: pc
type: ref
possible_keys: fk_pc_pb
key: fk_pc_pb
key_len: 4
ref: readyinteractive.pb.PromotionID
rows: 249
Extra: Using where
*************************** 14. row ***************************
id: 6
select_type: UNION
table: ppt
type: index
possible_keys: PRIMARY
key: ppt_pt
key_len: 4
ref: NULL
rows: 1
Extra: Using index
*************************** 15. row ***************************
id: 6
select_type: UNION
table: pb
type: eq_ref
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: readyinteractive.ppt.PromotionID
rows: 1
Extra:
*************************** 16. row ***************************
id: 6
select_type: UNION
table: pc
type: ref
possible_keys: fk_pc_pb
key: fk_pc_pb
key_len: 4
ref: readyinteractive.pb.PromotionID
rows: 249
Extra: Using where
*************************** 17. row ***************************
id: NULL
select_type: UNION RESULT
table: <union2,3,4,5,6>
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra:
17 rows in set (0.18 sec)