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.
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.