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Re: primary key vs covered index
Posted by: Øystein Grøvlen
Date: December 01, 2016 03:57AM

marcin kot Wrote:
-------------------------------------------------------
> I would like to get all columns from the index
> dokumenty_multi_i2 without column PLIK mediumblob.
> How to rewrite query or reindex table?

Hi,

If the BLOB is large, it will not be stored within the ordinary data page. Hence, I do not see that there should be any performance benefit from using another index than the PRIMARY here. You can of course force the query to use the other index:

SELECT lp,opis,naz ,dat_dod,oso_dod
FROM dokumenty FORCE INDEX(dokumenty_multi_i2)
WHERE id_dok=37399;

However, I do not believe this will run any faster.

Øystein Grøvlen,
Senior Principal Software Engineer,
MySQL Group, Oracle,
Trondheim, Norway

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