Date: Mon, 30 Nov 2020 09:29:02 +0100
From: David Fetter <david@fetter.org>
To: "tsunakawa.takay@fujitsu.com" <tsunakawa.takay@fujitsu.com>
Cc: 'Tomas Vondra' <tomas.vondra@enterprisedb.com>,
	'Tomas Vondra' <tomas.vondra@2ndquadrant.com>,
	"Andrey V. Lepikhov" <a.lepikhov@postgrespro.ru>,
	"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: POC: postgres_fdw insert batching
Message-ID: <20201130082902.GD9795@fetter.org>
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On Wed, Nov 25, 2020 at 05:04:36AM +0000, tsunakawa.takay@fujitsu.com wrote:
> From: Tomas Vondra <tomas.vondra@enterprisedb.com>
> > On 11/24/20 9:45 AM, tsunakawa.takay@fujitsu.com wrote:
> > > OTOH, as for the name GetModifyBatchSize() you suggest, I think
> > GetInsertBatchSize may be better.  That is, this API deals with multiple
> > records in a single INSERT statement.  Your GetModifyBatchSize will be
> > reserved for statement batching when libpq has supported batch/pipelining to
> > execute multiple INSERT/UPDATE/DELETE statements, as in the following
> > JDBC batch updates.  What do you think?
> > >
> > 
> > I don't know. I was really only thinking about batching in the context
> > of a single DML command, not about batching of multiple commands at the
> > protocol level. IMHO it's far more likely we'll add support for batching
> > for DELETE/UPDATE than libpq pipelining, which seems rather different
> > from how the FDW API works. Which is why I was suggesting to use a name
> > that would work for all DML commands, not just for inserts.
> 
> Right, I can't imagine now how the interaction among the client, server core and FDWs would be regarding the statement batching.  So I'll take your suggested name.
> 
> 
> > Not sure, but I'd guess knowing whether batching is used would be
> > useful. We only print the single-row SQL query, which kinda gives the
> > impression that there's no batching.
> 
> Added in postgres_fdw like "Remote SQL" when EXPLAIN VERBOSE is run.
> 
> 
> > > Don't worry about this, too.  GetMaxBulkInsertTuples() just returns a value
> > that was already saved in a struct in create_foreign_modify().
> > >
> > 
> > Well, I do worry for two reasons.
> > 
> > Firstly, the fact that in postgres_fdw the call is cheap does not mean
> > it'll be like that in every other FDW. Presumably, the other FDWs might
> > cache it in the struct and do the same thing, of course.
> > 
> > But the fact that we're calling it over and over for each row kinda
> > seems like we allow the value to change during execution, but I very
> > much doubt the code is expecting that. I haven't tried, but assume the
> > function first returns 10 and then 100. ISTM the code will allocate
> > ri_Slots with 25 slots, but then we'll try stashing 100 tuples there.
> > That can't end well. Sure, we can claim it's a bug in the FDW extension,
> > but it's also due to the API design.
> 
> You worried about other FDWs than postgres_fdw.  That's reasonable.  I insisted in other threads that PG developers care only about postgres_fdw, not other FDWs, when designing the FDW interface, but I myself made the same mistake.  I made changes so that the executor calls GetModifyBatchSize() once per relation per statement.

Please pardon me for barging in late in this discussion, but if we're
going to be using a bulk API here, wouldn't it make more sense to use
COPY, except where RETURNING is specified, in place of INSERT?

Best,
David.
-- 
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778

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