Horton

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

James Coleman <jtc331@gmail.com>

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From: James Coleman <jtc331@gmail.com>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Bruce Momjian <bruce@momjian.us>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2019-09-29T01:54:48Z
Lists: pgsql-hackers 
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

Commits

  1. Call out vacuum considerations in create index docs

  2. Document concurrent indexes waiting on each other

  3. snapshot scalability: Don't compute global horizons while building snapshots.

  4. Remove PROC_IN_ANALYZE and derived flags

  5. Improve performance of get_actual_variable_range with recently-dead tuples.