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Re: making a row unique
Posted by: Rick James
Date: February 24, 2009 11:45AM

Review of some INDEX rules that I think both of you are confused about...
* 0-1 PRIMARY KEY on a table.
* Any number of UNIQUE keys on a table.
* PRIMARY KEY (a,b) does not imply that 'a' is unique. If it is, you probably want PRIMARY KEY (a) plus INDEX (a,b).
* "KEY" == "INDEX".

And some guidelines:
* If you have INDEX(a,b), then it "covers" INDEX(a), and you can probably do without the latter.
* Prefixing is usually not useful: INDEX(a(10))
* Prefixing with UNIQUE is usually wrong: UNIQUE(a(10)) constrains the first 10 chars of 'a' to be UNIQUE.
* An AUTO_INCREMENT field should be the PRIMARY KEY. (Yeah, there are other tricks; this is a guideline.)
* In InnoDB, you should provide a PRIMARY KEY.
* In InnoDB, the PRIMARY KEY should be short if you have secondary keys.

Performance hints:
* More indexes means more work on inserts.
* If all the fields needed for a SELECT are found in a single index, then you get a performance boost. EXPLAIN will indicate this with "Using index".
* INDEX(a,b) is useless if you only say WHERE b=123.
* INDEX(a,b) is not as good as (b,a) when you have a>123 AND b=234. That is, want to start with "=".
* foo LIKE '%abc' will not use INDEX(foo), but this will: foo LIKE 'abc%'.

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February 23, 2009 12:12PM
February 23, 2009 12:20PM
February 23, 2009 02:11PM
February 23, 2009 02:41PM
Re: making a row unique
February 24, 2009 11:45AM
February 24, 2009 03:03PM
February 24, 2009 09:37PM
February 23, 2009 12:33PM
February 23, 2009 12:56PM
February 23, 2009 01:00PM
February 23, 2009 01:02PM

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