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help / color / mirror / Atom feedFrom: Ron Johnson <[email protected]>
To: David Rowley <[email protected]>
Cc: pgsql-general <[email protected]>
Subject: Re: CLUSTER vs. VACUUM FULL
Date: Sun, 21 Apr 2024 22:50:46 -0400
Message-ID: <CANzqJaDRsY7POw=-90bGzkTjJRa6N-_wQRgEPrNM34phiUjVLQ@mail.gmail.com> (raw)
In-Reply-To: <CAApHDvraS7EkYZsgZnAWc-L4_jbie5UkPAtedKSqN6s1j+PJiQ@mail.gmail.com>
References: <CANzqJaD8o0UuK42F==BXZA=L380n=aXt+VJ93PN3waFbNhZHBg@mail.gmail.com>
<[email protected]>
<CANzqJaBvcGKdhqaZz+b0jNc8WLK74UCDW+zk=Ja78duOjWUBRA@mail.gmail.com>
<CAApHDvraS7EkYZsgZnAWc-L4_jbie5UkPAtedKSqN6s1j+PJiQ@mail.gmail.com>
On Sun, Apr 21, 2024 at 9:35 PM David Rowley <[email protected]> wrote:
> On Mon, 22 Apr 2024 at 12:16, Ron Johnson <[email protected]> wrote:
> >
> > On Sun, Apr 21, 2024 at 6:45 PM Tom Lane <[email protected]> wrote:
> >>
> >> Ron Johnson <[email protected]> writes:
> >> > Why is VACUUM FULL recommended for compressing a table, when CLUSTER
> does
> >> > the same thing (similarly doubling disk space), and apparently runs
> just as
> >> > fast?
> >>
> >> CLUSTER makes the additional effort to sort the data per the ordering
> >> of the specified index. I'm surprised that's not noticeable in your
> >> test case.
> >
> > Clustering on a completely different index was also 44 seconds.
>
> Both VACUUM FULL and CLUSTER go through a very similar code path. Both
> use cluster_rel(). VACUUM FULL just won't make use of an existing
> index to provide presorted input or perform a sort, whereas CLUSTER
> will attempt to choose the cheapest out of these two to get sorted
> results.
>
> If the timing for each is similar, it just means that using an index
> scan or sorting isn't very expensive compared to the other work that's
> being done. Both CLUSTER and VACUUM FULL require reading every heap
> page and writing out new pages into a new heap and maintaining all
> indexes on the new heap. That's quite an effort.
>
My original CLUSTER command didn't have to change the order of the data
very much, thus, the sort didn't have to do much work.
CLUSTER on a different index was indeed much slower than VACUUM FULL.
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Subject: Re: CLUSTER vs. VACUUM FULL
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