Thread: Update table is much faster after copying it

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
attributeof 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
usingCREATE 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 around with using CLUSTER but it doesn’t seem to have
anyeffect, and I don’t really see why it should given that table a needs a sequential scan to update every row anyway.
 

Thanks for your insights.

-Miles

Miles Jordan <miles.jordan@rea-group.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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