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

On Sat, Sep 28, 2019 at 09:54:48PM -0400, James Coleman wrote:
> 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.

I think any open transaction will block CIC, which is my point.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

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