Comments on: Using UNION to implement loose index scan in MySQL

By: Spyd

Spyd — Fri, 10 Feb 2012 15:53:51 +0000

Hey, i found this page while googling for more information after i implemented something exactly like this! I thought i had done something “dirty” but now i’m happy that this “tricky” method is suggested by you! My queries went from 0,6 to 0,01

By: Jindra

Jindra — Thu, 21 Jan 2010 17:38:47 +0000

Thanks, helped me to optimize query with agregate function on table with multiple key:

table:
Create table tabData (
channel_id Int NOT NULL,
date_time Datetime NOT NULL,
value Double NOT NULL,
Primary Key (channel_id, date_time)
);

too slow (more then a few seconds when thousands of records):

select min(date_time) from tabData where id_virtual_channel in (1, 2, 3)

optimized – query returns immediately:

select min(m)
from (
select min(date_time) as m from tabData where channel_id = 1
union
select min(date_time) from tabData where channel_id = 2
union
select min(date_time) from tabData where channel_id = 3
) as min_datetimes

By: How to select the first/least/max row per group in SQL | Wilson80s

How to select the first/least/max row per group in SQL | Wilson80s — Wed, 11 Nov 2009 20:15:31 +0000

[...] Peter Zaitsev has written in detail about this technique, so I wonâ€™t go into it too much more here. If it suits your purposes, it can be a very good solution. [...]

By: Mark

Mark — Thu, 15 Oct 2009 00:34:32 +0000

Its 3 years later, has this been fixed? If so, in what version of MySQL?

Thanks!

By: How to select the first/least/max row per group in SQL at Xaprb

How to select the first/least/max row per group in SQL at Xaprb — Fri, 08 Aug 2008 15:39:39 +0000

[...] Peter Zaitsev has written in detail about this technique, so I won’t go into it too much more here. If it suits your purposes, it can be a very good solution. [...]

By: How adding another table to JOIN can improve performance ? | MySQL Performance Blog

Sat, 02 Aug 2008 05:10:59 +0000

[...] have described couple of solutions to this problem – using IN list instead of range or UNION which however require [...]

By: James Day

James Day — Wed, 16 Aug 2006 18:30:54 +0000

Apachez, one of many areas where it’s known that the optimizer can be improved. They are gradually being done, no particular schedule for when each one is done. Other than more slowly than we’d all like.

James Day
Support Engineer, MySQL AB

By: MySQL Performance Blog » MySQL: Followup on UNION for query optimization, Query profiling

Mon, 14 Aug 2006 20:46:32 +0000

[...] Few days ago I wrote an article about using UNION to implement loose index scan. [...]

By: Apachez

Apachez — Mon, 14 Aug 2006 18:32:29 +0000

Is this a MySQL Optimizer bug (which we could expect to be fixed in near future) or is this “by design” for the next couple of years ?

I mean the UNION is a nice workaround but needs more code and logic where just writing a “WHERE col BETWEEN 19 AND 22″ is so much easier

By: peter

peter — Sun, 13 Aug 2006 15:52:08 +0000

Yuan WANG,

If query with where clause “age=19 and zip in (…)” is executed what really is performed is sequence of lookups
age=19 and zip=const. Key length=4 is missleading in this case. It seems to be EXPLAIN bug in this case.

For query with “age between 19 and 22 and zip in (…)” what you have described is happening – index range lookup is performend on age condition and zip condition only checked after row is read.

You can use FLUSH STATUS; SHOW STATUS; and check Handler_XXX stats to see how query execution was performed.