Alvaro Herrera wrote:
You can use CLUSTER reliably only from 7.2 upwards. (Or was it 7.3? I
forget). In earlier versions it would lose information about other
indexes (i.e. those not being clustered on), foreign keys, inheritance,
etc; in other words pretty much a disaster except for the simplest of
tables.
Interesting historical note, but fortunately largely irrelevant these days.
Also, it is MVCC-safe only from 8.3 upwards; on older versions
it (incorrectly) deletes dead tuples that are still visible to old
transactions.
More interesting. I may have a broken mental-model. I *thought* that
CLUSTER acquired exclusive locks and that acquisition of the exclusive
lock would imply that there couldn't be any transactions accessing that
table. Where is my misunderstanding?
Of course, the main problem with CLUSTER is that it needs about 2x the
disk space of table + indexes.
Again checking my mental model. My understanding is that CLUSTER
basically recreates the tables and indexes and then swaps the new ones
in place of the originals. So ~2x is true for typical tables. But for
tables badly bloated by multiple bulk updates or bad vacuum practices
CLUSTER should require far less than 2x.
Cheers,
Steve
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