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

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.

--
Álvaro Herrera https://www.2ndQuadrant.com/
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