Re: WIP: hooking parser

2009/2/16 Sam Mason <sam@samason.me.uk>:
> On Mon, Feb 16, 2009 at 08:03:42PM +0100, Pavel Stehule wrote:
>> 2009/2/16 Sam Mason <sam@samason.me.uk>:
>> > But to do it properly inside PG would be difficult; how would your hooks
>> > know to transform:
>> >
>> >  SELECT s FROM foo WHERE s IS NULL;
>> >
>> > into:
>> >
>> >  SELECT s FROM foo WHERE (s = '' OR s IS NULL);
>>
>> I don't need it. Oracle store NULL without ''. So expression some IS
>> NULL is stable.
>
> OK, I was under the impression that you wanted general Oracle
> compatibility from PG.  Apparently this isn't the case.
>
>> > that all looks a bit tricky to me.  Hum... actually it's not.  All you
>> > need to do is to rewrite any string reference "s" into NULLIF(s,'').
>> > That would tank performance as indexes wouldn't be used most of the
>> > time, but never mind.
>>
>> look to source what I do. It' just simple. But you have to emulate
>> Oracle behave everywhere. Then all is simple, because Oracle doesn't
>> know ''.
>
> Yes, I read your code.  You'll still get zero length strings back from
> things like substring('hello world',1,0) and not a NULL as I expect
> you'd get back from Oracle.
>

my sample is very simple - full emulation needs maybe 100 lines more,
but it is possible. After finishing transformation is possible to get
rusult type and I can do some really easy alchemy and wrap funccall
for some text functions and replace simple string with NULL. Similar
game is playing now when you use variadic function or function with
defaults arguments.

>
> [ context removed by Pavel; but the example was supporting Informix
> style named parameters by PG ]
>
>> > That would be a *much* bigger change; you're actually changing PG's
>> > parser there and not just modifying the parse tree.  If it was done
>> > externally it would be a much easier thing to do.
>>
>> No I don't do it. Loadable modules are really external. I need only
>> hook inside parser.
>
> As far as I checked, your code gets passed some subset of the parse
> tree.  For the parser to have a chance of getting the code to your hook
> it would need to be considered valid syntax.  Informix style named
> parameters isn't considered valid by PG's parser and hence the user will
> get an error before the hook would get a chance to rewrite the parse
> tree and make it valid.  This is basically what Tom was alluding to
> here:
>

Sure. I need some basic functionality, PostgreSQL have to support
named params. But for example, Informix style named params are valid
now (for bison stage).

There are two etaps - Bison parsing - and transformation. And with
wrapping transformation I am able do it. Is paradox so I am able to do
it with Oracle or Informix syntax and not with planned PostgreSQL
syntax now (in this moment). When I find some functionality, that I
can use, then module is really simple - like decode implementation. It
is only transformation to specific CASE statement (specific, because I
have to use IS NOT DISTINCT operator). But without this base
functionality, I should to use C functions. It is only some code more.

This solution isn't absolutely general - It's not able emulate full
SQL/XML syntax - but current func_call rules are very simple. On
second hand It can support smart functions, that knows their source -
like some SQL/XML functions does.


>  http://archives.postgresql.org/pgsql-hackers/2009-02/msg00574.php
>
> --
>  Sam  http://samason.me.uk/
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