问题
I'm trying to optimize a query which uses a view in MySQL 5.1. It seems that even if I select 1 column from the view it always does a full table scan. Is that the expected behaviour?
The view is just a SELECT "All Columns From These Tables - NOT *" for the tables I have specified in the first query below.
This is my explain output from when i select the indexed column PromotionID from the query which makes up the view. As you can see it is vastly different from the output on the view.
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)
The output when i select the same thing but from the view
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)
回答1:
Views in MySQL are not indexed so by their very nature require a full scan each time they are accessed. Generally speaking this makes Views really only useful for situations where you have a fairly complex static query that returns a small result set and you plan to grab the entire result set every time.
Edit: Of course Views will use the indexes on the underlying tables so that the View itself is optimized (otherwise they wouldn't make any sense at all to use) but because there are no indexes on a View it is not possible for a WHERE query on the View to be optimized.
Constructing indexes for Views would be expensive anyway because while I've not tried to profile any Views, I'm fairly certain that a temp table is constructed behind the scenes and then the result set returned. It already takes plenty of time to construct the temp table, I wouldn't want a view that also tries to guess what indexes are needed. Which brings up the second point which is that MySQL does not currently offer a method to specify what indexes to use for a View so how does it know what fields need to be indexed? Does it guess based on your query?
You might consider using a Temporary Table because then you can specify indexes on fields in the temporary table. However, from experience this tends to be really, really slow.
If all this view contains is a SELECT ALL FROM table1, table2, table3; then I would have to ask why this query needs to be in a View at all? If for some reason its absolutely necessary, you might want to use a stored procedure to encapsulate the query as you'll then be able to get optimized performance while maintaining the benefit of a simpler call to the database for the result set.
回答2:
I've looked deeper into it an I've missed a key point of information :( My view query actually has a union with another table. This is causing the view to use the TEMPORARY TABLE algorithm instead of the MERGE algorithm.
The TEMPORARY TABLE algorithm doesn't allow the use of indexes in the underlying tables.
This seems to be a bug in MySQL and was reported way back in 2006 but doesn't look like it has been solved in 2009! http://forums.mysql.com/read.php?100,56681,56681
Looks like i'm just going to have to re-write the query as an outer join.
来源：https://stackoverflow.com/questions/637328/does-mysql-view-always-do-full-table-scan