Comments on: 3 ways MySQL uses indexes

By: Ted Naleid » Batch Import Performance with Grails and MySQL

Ted Naleid » Batch Import Performance with Grails and MySQL — Thu, 01 Oct 2009 05:54:01 +0000

[...] you haven’t created many indexes before, I highly suggest reading this post on indexes on the mysql high performance blog. The executive summary is that multi-key indexes tailored to your query have a performance edge [...]

By: peter

peter — Mon, 21 Sep 2009 17:43:30 +0000

Jeff,

Thanks. Great to hear our articles are are helpful for you.

By: Jeff

Jeff — Sun, 20 Sep 2009 23:59:27 +0000

I had been working on a performance problem for a while and this article triggered a new way for me to think about it and ultimately helped me resolve it. Thanks so much for this article in particular but also all of your articles. Some really great stuff!

By: Weekly Link Dump #1 | Dmytro Shteflyuk's Home

Weekly Link Dump #1 | Dmytro Shteflyuk's Home — Mon, 14 Sep 2009 09:49:28 +0000

[...] 3 ways MySQL uses indexes [...]

By: peter

peter — Mon, 14 Sep 2009 05:13:04 +0000

Right,

I think key_len is rather confusing, especially as it is length in bytes rather than key parts. This makes this complicated because of different length of different data types. For example if you have key (A,B) and see key_len=8 it may be using full index with INT fields or only A prefix for BIGINT column.

By: zerkms

zerkms — Mon, 14 Sep 2009 03:30:21 +0000

“I speak later in the post you should look at key_len to see how many key parts actually used to lookup rows”
In this case i hate mysql and like mssql, because mssql show what (and even how!!) index parts were used.

By: peter

peter — Sun, 13 Sep 2009 18:05:02 +0000

Zerkms,

This is actually 2 expressions one A BETWEEN 5 AND 10 and second one A=5 AND B BETWEEN 5 AND 10. Sorry for layout it is not always clear. Indeed as I speak later in the post you should look at key_len to see how many key parts actually used to lookup rows. in cases like A BETWEEN 5 AND 10 AND B BETWEEN 5 AND 10 the index will be used but really only for first part of the predicate using only (A) key prefix.

By: zerkms

zerkms — Sun, 13 Sep 2009 09:30:16 +0000

sorry, i was not enought intent. please delete my 2 posts :-[

By: zerkms

zerkms — Sun, 13 Sep 2009 09:27:26 +0000

“So if you have index on (A,B) This index can be used to lookup rows for WHERE clauses like A=5 ; A BETWEEN 5 AND 10 A=5 AND B BETWEEN 5 AND 10″

The last part of sentense isn’t clearly right: for expression A BETWEEN 5 AND 10 A=5 AND B BETWEEN 5 AND 10, only the left part of A+B index will be used. i think this fact should be noticed or sample changed to be more “simple”.

or i’m getting wrong?

By: gmouse

gmouse — Sat, 12 Sep 2009 17:39:13 +0000

It would be nice to see a larger post or article which fully describes how indices are used, also for joins. I still encounter too many people who are clueless, and it would be nice if I could provide them with a full article.