On Mon, 22 Apr 2024 at 12:16, Ron Johnson <ronljohnsonjr@gmail.com> wrote:
>
> On Sun, Apr 21, 2024 at 6:45 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>
>> Ron Johnson <ronljohnsonjr@gmail.com> 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.
To satisfy your curiosity, you could always run some EXPLAIN ANALYZE
SELECT queries to measure how much time was spent sorting the entire
table. You'd have to set work_mem to the value of
maintenance_work_mem.
David