Re: pgbench more operators & functions

From: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
To: Stephen Frost <sfrost(at)snowman(dot)net>
Cc: Jeevan Ladhe <jeevan(dot)ladhe(at)enterprisedb(dot)com>, PostgreSQL Developers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: pgbench more operators & functions
Date: 2016-10-01 14:35:26
Message-ID: alpine.DEB.2.20.1610011500320.14960@lancre
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Hello Stephen,

>> bitwise: <<, >>, &, |, ^/#, ~
>> comparisons: =/==, <>/!=, <, <=, >, >=
>> logical: and/&&, or/||, xor/^^, not, !
> I'm not sure that we want to introduce operators '&&', '||' as logical
> 'and' and 'or' when those have specific meaning in PG which is different
> (array overlaps and concatenation, specifically). I can certainly see
> how it could be useful to be able to perform string concatenation in a
> \set command in pgbench, for example..
> Also, are we sure that we want to have both '=' and '==' for comparison?

The reason I added C operators is that Pg SQL has '!=' and a few others,
so once some are there I do not see why others should not be there as well
for consistency.

Now I agree that having operators which behave differently from psql to
pgbench is a bad idea.

In the attached patched I only included pg operators, plus "xor" which I
feel is missing and does not seem to harm.

> [...] I certainly understand how the if() function could be useful

Indeed, some kind of "if" is needed, for instance to implement "tpc-b"

> , but I'm not entirely sure that the if(expression, true-result,
> false-result) is the best approach. In particular, I recall cases with
> other languages where that gets to be quite ugly when there are multiple
> levels of if/else using that approach.

I think that pgbench is not a programming language, but an expression
language, which means that you have to provide a result, so you can only
have balanced if/then/else, which simplifies things a bit compared to
"full" language.

The SQL syntax for CASE is on the very heavy side and would be quite
complicated to implement in pgbench, so I rejected that and selected the
simplest possible function for the job.

Maybe the "if" function could be named something fancier to avoid possible
future clash on an eventual "if" keyword? Note that excel has an IF
function. Maybe "conditional"... However, as I do not envision pgbench
growing to a full language, I would be fine keeping "if" as it is.

> The table should really have a row per operator, which is what we do in
> pretty much all of the core documention. [...]

Ok for one line per operator.

> The question which was brought up about having a line-continuation
> (eg: '\') character would be useful,

:-) I submitted a patch for that, which got rejected and resulted in Tom
making pgbench share psql lexer. This is a good thing, although the
continuation extension was lost in the process. Also this means that now
such logic would probably be shared with psql, not necessarily a bad thing
either, but clearly material for another patch.

> which really makes me wonder if we shouldn't try to come up with a way
> to create functions in a pgbench script that can later be used in a \set
> command.

I do not think that pgbench script is a good target to define functions,
because the script is implicitely in a very large loop which is executing
it over and over again. I do not think that it would make much sense to
redefine functions on each transaction... So my opinion is that without a
compeling use case, I would avoid such a feature, which would need some
careful thinking on the design side, and the implementation of which is
non trivial.

> With such an approach, we could have proper control structures
> for conditionals (if/else), loops (while/for), etc, and not complicate
> the single-statement set of operators with those constructs.

If you want control, you can already use a DO & PL/pgsql script, although
it is interpreted server-side. Yet again, I'm not convince that pgbench is
material for such features, and it would take a compeling use case for me
to add such a thing. Moreover, the currently simple internal data
structures and executor would have to be profoundly reworked and extended
to handle a full language and functions.

> Lastly, we should really add some regression tests to pgbench. We
> already have the start of a TAP test script which it looks like it
> wouldn't be too hard to add on to.

I agree that pgbench should be tested systematically. I think that this is
not limited to these functions and operators but a more general and
desirable feature, thus is really material for another patch.

Attached version changes:
- removes C operators not present in psql
- document operators one per line


Attachment Content-Type Size
pgbench-more-ops-funcs-5.patch text/x-diff 16.5 KB

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