Re: [DOC] Document concurrent index builds waiting on each other

On Sat, Sep 28, 2019 at 9:22 PM Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
>
> On 2019-Sep-28, Bruce Momjian wrote:
>
> > The CREATE INDEX docs already say:
> >
> >     In a concurrent index build, the index is actually entered into
> >     the system catalogs in one transaction, then two table scans occur in
> >     two more transactions.  Before each table scan, the index build must
> >     wait for existing transactions that have modified the table to terminate.
> >     After the second scan, the index build must wait for any transactions
> > --> that have a snapshot (see <xref linkend="mvcc"/>) predating the second
> > --> scan to terminate.  Then finally the index can be marked ready for use,
> >
> > So, having multiple concurrent index scans is just a special case of
> > having to "wait for any transactions that have a snapshot", no?  I am
> > not sure adding a doc mention of other index builds really is helpful.
>
> I always thought that create index concurrently was prevented from
> running concurrently in a table by the ShareUpdateExclusive lock that's
> held during the operation.

You mean multiple CICs on a single table at the same time? Yes, that
(unfortunately) isn't possible, but I'm concerned in the patch with
the fact that CIC on table X blocks CIC on table Y.

James