Comments on: Derived Tables and Views Performance

By: John Larsen

John Larsen — Thu, 15 Jan 2009 16:34:00 +0000

Baron: You know what your right. Somehow in the past I was able to get a view to do this, think it was for a aggregated value (view was an aggregate) which was then trimmed down in queries to the view to be pieces of the aggregate. Was able to get better performance at the time on multiple queries in a row (first query of a connection was slow later queries were instant) then a straight query to the table as the view was holding the aggregate data used for selection of its parts. Can’t seem to figure out how I did it now as I didn’t specify any special notation to it (like TEMPTABLE).

By: Baron Schwartz

Baron Schwartz — Thu, 15 Jan 2009 14:37:38 +0000

John, that is incorrect. Views do not store their data. You are mixing the TEMPTABLE algorithm (which creates an internal temp table for the duration of the query) and the query cache (which stores result sets until underlying tables change or other conditions require discarding them). And views only permit usage of the underlying table’s indexes if the MERGE algorithm is used.

By: John Larsen

John Larsen — Thu, 15 Jan 2009 04:33:33 +0000

Ari: Did you try immediently running it again with a different where this is where views shine. Yes unless specifically made not to a view will run the exact select stored in the view. But untill the table it is based upon changes relative to that view a temporary table will be stored containing the full view. As the view temp table won’t need recreating a second run with where would be faster than the second run of a regular select because you have less columns to deal with. A view will generally retain it parents indexes.

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By: javatopia

javatopia — Fri, 14 Dec 2007 22:18:14 +0000

We ran into this problem just recently on a real world query. In this case the query was of this form:

select * from (big inner query) foo order by a, b, c

The choice to use this query syntax was for programming brevity – it’s easier to rename the output columns from the big inner query using the outer query instead. Alas, though, the nasty version of this query took over 2.5 minutes to run on 53,000 record scans. The version without the outer select ran in 9/10th of a second on the same record space.

The EXPLAIN clearly showed that the outer select was doing a full table scan on the results instead of using the indices.

Unbelievable. I ran the exact same nasty query in Microsoft SQL Server 2000 and it performed as expectd – lightning fast execution. The execution plan showed the effective use of table indices and much smarter insertion of the sub-query’s grouping clauses.

MySQL needs quite a bit of real-world tuning to make it a believable enterprise database. You get what you pay for, right?

By: ari

ari — Fri, 18 May 2007 05:48:27 +0000

we are currently using mysql 5.037(27)
it looks like when you create a view on a table and then use this view in query with a join or where statement with another table indexes
from a view(first original table) are not used.Run the same query replacing view with a table it is based and result is in several times faster.

By: 派生表和视图的性能 at 周俭 Blog

派生表和视图的性能 at 周俭 Blog — Sat, 16 Sep 2006 14:03:18 +0000

[...] 原文作者： Peter Zaitsev原文来源： http://www.mysqlperformanceblog.com/2006/08/31/derived-tables-and-views-performance译者：叶金荣（Email:），转载请注明译者和出处，并且不能用于商业用途，违者必究。 [...]

By: Martijn Tonies

Martijn Tonies — Mon, 04 Sep 2006 10:01:58 +0000

Interesting, thanks for trying.

–
Martijn Tonies
Upscene Productions

By: peter

peter — Fri, 01 Sep 2006 20:14:37 +0000

Martijn,

First – you’re right. There is no reason for derived tables to be materialized. It is design deficiency in MySQL 5.0.

Speaking about Views with group by – good question. MySQL is currently able to execute via query merge only simple views, more complicated views as ones with group by require temporary table even if it is possible to avoid it. Hopefully it will be improved in the future:

mysql> create view v1 as select count(*) cnt, k from test group by k;
Query OK, 0 rows affected (0.03 sec)

As you see full temporary table is used, there is index on k but it only allows to do index scan instead of full table scan.

By: Martijn Tonies

Martijn Tonies — Fri, 01 Sep 2006 18:26:26 +0000

I wonder what happens if you use a view that cannot be easily rewritten.

eg: GROUP BY or have a SUM in it.

Now, do a “select * from view where summed_column = 50″ or something.

Will this be re-written? And if so, what IS the final result of the re-written query?

Besides, why DO derived tables get materialized anyway? The optimizer is free to re-write queries including these as well, right?

–
Martijn Tonies
Upscene Productions