Re: Update table is much faster after copying it

Miles Jordan <miles(dot)jordan(at)rea-group(dot)com> wrote:

> Hi,
>
> I have tables a and b each with around 12 million rows. I am running
> an update query to set the id of table b as an attribute of table a,
> and have an appropriate index on table b that EXPLAIN tells me should
> be used.
>
> The query takes 6 hours on an AWS db.r3.4xlarge. That seemed hugely
> excessive, so I copied table a into a new table using CREATE TABLE b
> AS SELECT * FROM a, and for completeness also added the same indexes
> and constraints.
>
> Now, when I run the update on table a, it finishes in 3 minutes, and
> produces the same result.
>
> Can anyone shed some light on why this might happen? I’ve played

there are 2 possible reasons: caching and maybe a lot of dead tuples
within the original table. Please check the on-disk-size of table a and
b.

Andreas
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