Date: Thu, 14 Jul 2016 10:02:18 +0200 (CEST)
From: Fabien COELHO <coelho@cri.ensmp.fr>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: Robert Haas <robertmhaas@gmail.com>,
        Pavel Stehule <pavel.stehule@gmail.com>,
        PostgreSQL Developers <pgsql-hackers@postgresql.org>
Subject: Re: pgbench - allow to store select results into
 variables
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Hello Tom,

>>> SELECT 1, 2 \; SELECT 3;
>>> \into one two three
>
> Yeah, that's seriously nasty action-at-a-distance in my view.  I'd be okay
> with
>
> SELECT 1, 2 \into one two
> SELECT 3 \into three

ISTM that is not the same, because then you would have two queries (over 
the network) instead of one, so you pay the network latency twice?

> but I do not think that a metacommand on a following line should
> retroactively affect the execution of a prior command, much less commands
> before the last one.

Nope. The meta-command applies to the preceeding SQL command... which 
happens to be a \;-compound command. ISTM that all is logically fine.


Some motivation about the feature (not its syntax or implementation), from 
a benchmarking perspective:

- clients MUST read the server answers and possibly reuse them, hence a 
proposed \into feature. Discarding the answer as pgbench does not really 
comply with typical benchmark rules, eg from tpc-b:

   """1.3.2 Each transaction shall return to the driver the Account_Balance
      resulting from successful commit of the transaction.

   Comment: It is the intent of this clause that the account balance in the
   database be returned to the driver, i.e., that the application retrieve
   the account balance."""

- latency is important to applications (eg web applications), thus the 
ability to compound statements is a good thing. However, if in a bench one 
can compound statements but not retrieve their values, it fails the 
previous "retrieve the value" requirement.

So basically I wish to avoid splitting compound queries and paying latency 
just because of a lack of syntax to do the right thing, hence the proposed 
feature which can retrieve data from various parts of a compound 
statement.

-- 
Fabien.


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