Re: [GENERAL] Using an SMP machine to make multiple indices on the same table

Martin Weinberg <weinberg(at)osprey(dot)astro(dot)umass(dot)edu> writes:
> Yes, I understand locking the table, but empirically, two index
> creations will not run simultaneously on the same table.

Hmm, on trying it you are right. The second index creation blocks here:

#6 0x1718e0 in XactLockTableWait (xid=17334) at lmgr.c:344
#7 0x9e530 in heap_mark4update (relation=0xc1be62f8, tuple=0x7b03b7f0,
buffer=0x7b03b828) at heapam.c:1686
#8 0xcb410 in LockClassinfoForUpdate (relid=387785, rtup=0x7b03b7f0,
buffer=0x7b03b828, confirmCommitted=0 '\000') at index.c:1131
#9 0xcb534 in IndexesAreActive (relid=387785, confirmCommitted=1 '\001')
at index.c:1176
#10 0xf0f04 in DefineIndex (heapRelationName=0x400aab20 "tenk1",
indexRelationName=0x400aab00 "anotherj", accessMethodName=0x59f48 "btree",
attributeList=0x400aab80, unique=0, primary=0, predicate=0x0,
rangetable=0x0) at indexcmds.c:133
#11 0x17e118 in ProcessUtility (parsetree=0x400aaba0, dest=Remote)
at utility.c:905

Essentially it's trying to do a SELECT FOR UPDATE on the pg_class tuple
of the relation before it starts building the index.

I have opined before that LockClassinfoForUpdate is a mistake that
shouldn't exist at all, since acquiring the proper lock on the relation
ought to be sufficient. I see no need for locking the pg_class tuple,
and certainly none for doing so at the beginning of the operation rather
than the end.

Hiroshi, I think you defended it last time; any comments?

regards, tom lane