Re: Is full-row updates slower than single-value updates

Hello

2010/6/28 Björn Lindqvist <bjourne@gmail.com>:
> Hello everyone,
>
> My question is like the subject, is it much slower to update all
> columns values than just a single column? Generated update queries
> from ORM:s generally have the following format:
>
> update foo set a = 1, b = 2, c = 3, .... where id = 1234;
>
> So each column is touched which, I suspect, is much slower than just
> updating a single column because more indexes has to be rebuilt and so
> on. EXPLAIN ANALYZE does not give any hints and claims that both
> queries are equally fast. Maybe someone can guide me to documentation
> which explains how to estimate how costly row-level updates are?
>

it depends. Pg create a new version of complete row for every update,
so isn't important if you update one or all columns. But there are
exception - TOAST columns. If you update any TOAST column, then UPDATE
is significally slower, so - if you don't need to update these
columns, then don't do it.

Regards

Pavel

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> mvh Björn
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