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

On Sat, Sep 28, 2019 at 9:56 PM Bruce Momjian <bruce@momjian.us> wrote:
>
> 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.

While that may be technically true, as a co-worker of mine likes to
point out, being "technically correct" is the worst kind of correct.

Here's what I mean:

First, I believe the docs should aim to be as useful as possible to
even those with more entry-level understanding of PostgreSQL. The fact
the paragraph you cite actually links to the entire chapter on
concurrency control in Postgres demonstrates that there's some
not-so-immediate stuff here to consider. For one: is it obvious to all
users that the transaction held by CIC (or even that all transactions)
has an open snapshot?

Second, this is a difference from a regular CREATE INDEX, and we
already call out as caveats differences between CREATE INDEX
CONCURRENTLY and regular CREATE INDEX as I point out below re:
Alvaro's comment.

Third, related to the above point, many DDL commands only block DDL
against the table being operated on. The fact that CIC here is
different is, in my opinion, a fairly surprising break from that
pattern, and as such likely to catch users off guard. I can attest
that this surprised at least one entire database team a while back :)
including many people who've been operating Postgres at a large scale
for a long time.

I believe caveats like this are worth calling out rather than
expecting users to have to understand the implementation details an
work out the implications on their own.

> > > 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.

I read Alvaro as referring to the fact that the docs already call out
the following:

> Regular index builds permit other regular index builds on the same table to occur simultaneously, but only one
concurrentindex build can occur on a table at a time.
 

James