Обсуждение: bind variables, soft vs hard parse

Hi list.

I've mostly used Oracle in the past, but for a web-project I took the 
opportunity to try Postgres.

When a select is done in Oracle, it first checks if the select is cached 
(ie parsed tree, optimizer choices & such). It does this by 
[functionality equal to] a byte to byte compare with the other sql strings.

select a from b where c = 1
select a from b where c = 2

...will thus force a hard parse on the second select. But if using bind 
variables it wont as the string stored is something like

select a from b where c = ?

Which will be the same as the second call. There is quite a big 
difference in performance using bind variables.

Does Postgres work the same? Where can I go for more info?

Oracle recently gave some money to Zend to make proper Oracle support 
for PHP. In that interface they use bind variables. Apart from greater 
speed, sqlinjection becomes history as well.

Best regards,
Marcus

Marcus Engene <mengpg@engene.se> writes:

> Which will be the same as the second call. There is quite a big
> difference in performance using bind variables.
>
> Does Postgres work the same? Where can I go for more info?

You can do this (or close to it) but you need to explicitly PREPARE
the query (or use the protocol-level prepare, which some client
libraries will do for you).  See the SQL documentation for PREPARE.

-Doug

PostgreSQL combines both parses into one, so every new query is
effectively a hard parse (unless it's prepared, then there is no parse
or optimization at all).

On Tue, Nov 15, 2005 at 07:33:46PM +0100, Marcus Engene wrote:
> Hi list.
> 
> I've mostly used Oracle in the past, but for a web-project I took the 
> opportunity to try Postgres.
> 
> When a select is done in Oracle, it first checks if the select is cached 
> (ie parsed tree, optimizer choices & such). It does this by 
> [functionality equal to] a byte to byte compare with the other sql strings.
> 
> select a from b where c = 1
> select a from b where c = 2
> 
> ...will thus force a hard parse on the second select. But if using bind 
> variables it wont as the string stored is something like
> 
> select a from b where c = ?
> 
> Which will be the same as the second call. There is quite a big 
> difference in performance using bind variables.
> 
> Does Postgres work the same? Where can I go for more info?
> 
> Oracle recently gave some money to Zend to make proper Oracle support 
> for PHP. In that interface they use bind variables. Apart from greater 
> speed, sqlinjection becomes history as well.
> 
> Best regards,
> Marcus
> 
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 6: explain analyze is your friend
> 

-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461

>      > Oracle recently gave some money to Zend to make proper Oracle support
>      > for PHP. In that interface they use bind variables. Apart from
>     greater
>      > speed, sqlinjection becomes history as well.

I did the same for PostgreSQL for PHP 5.1.

http://au3.php.net/manual/en/function.pg-query-params.php

Chris

Douglas McNaught wrote:
>>Which will be the same as the second call. There is quite a big
>>difference in performance using bind variables.
>>
>>Does Postgres work the same? Where can I go for more info?
> 
> You can do this (or close to it) but you need to explicitly PREPARE
> the query (or use the protocol-level prepare, which some client
> libraries will do for you).  See the SQL documentation for PREPARE.
> 
> -Doug

Hi,

But this is of no use in a web-context. According to the docs, this 
prepare is per session.

This sql cache I think is a really good thing. Is there a reason
Postgres hasn't got it? Would it be very hard to implement? From
a naive perspective; make a hashvalue from the sql-string to
quickly find the cached one, a "last used"-list for keeping
track of which to delete when cache full etc seems close to
trivial. Does the architecture/internal flow make it hard
actually reuse the query data structure?

Thanks for the answer.

Best regards,
Marcus

On Wed, Nov 16, 2005 at 09:56:44AM +0100, Marcus Engene wrote:
> Douglas McNaught wrote:
> >You can do this (or close to it) but you need to explicitly PREPARE
> >the query (or use the protocol-level prepare, which some client
> >libraries will do for you).  See the SQL documentation for PREPARE.
>
> But this is of no use in a web-context. According to the docs, this
> prepare is per session.

Unless you use something like pgpool, in which case a single session
may include multiple requests.

> This sql cache I think is a really good thing. Is there a reason
> Postgres hasn't got it? Would it be very hard to implement? From
> a naive perspective; make a hashvalue from the sql-string to
> quickly find the cached one, a "last used"-list for keeping
> track of which to delete when cache full etc seems close to
> trivial. Does the architecture/internal flow make it hard
> actually reuse the query data structure?

It's hard to reuse the structure. Also, things like search_path mean
that the same query text can mean completely different things in
different backends. Most of the time it's planning that dominates, not
parsing so storing just the parser output seems somewhat useless.

Unless you've thought of a new way to do it.

Have a nice day,
--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.

Christopher Kings-Lynne wrote:
>>      > Oracle recently gave some money to Zend to make proper Oracle 
>> support
>>      > for PHP. In that interface they use bind variables. Apart from
>>     greater
>>      > speed, sqlinjection becomes history as well.
> 
> 
> I did the same for PostgreSQL for PHP 5.1.
> 
> http://au3.php.net/manual/en/function.pg-query-params.php
> 
> Chris

Brilliant! I'll upgrade to 5.1 for this reason alone!

Best regards,
Marcus

Martijn van Oosterhout wrote:
>>But this is of no use in a web-context. According to the docs, this 
>>prepare is per session.
> 
> Unless you use something like pgpool, in which case a single session
> may include multiple requests.

ok. Good point.

>>This sql cache I think is a really good thing. Is there a reason
>>Postgres hasn't got it? Would it be very hard to implement? From
>>a naive perspective; make a hashvalue from the sql-string to
>>quickly find the cached one, a "last used"-list for keeping
>>track of which to delete when cache full etc seems close to
>>trivial. Does the architecture/internal flow make it hard
>>actually reuse the query data structure?
> 
> It's hard to reuse the structure. Also, things like search_path mean
> that the same query text can mean completely different things in
> different backends. Most of the time it's planning that dominates, not
> parsing so storing just the parser output seems somewhat useless.

Of course I didn't mean only the parse was to be saved. The planning
goes there too.

Thanks for the explanation.

> Have a nice day,

The same!

Marcus

Martijn van Oosterhout <kleptog@svana.org> writes:
> Unless you use something like pgpool, in which case a single session
> may include multiple requests.

Actually, I've found pgpool to be no better when it comes to using
real prepared queries---there's no guarantee that any given request is
going to connect to the same pgpool process as before, so it won't
have the prepared request.

Mike

Jim C. Nasby wrote:
> It might be more useful to look at caching only planning and not
> parsing. I'm not familiar with the output of the parsing stage, but
> perhaps that could be hashed to use as a lookup into a cache of planned
> queries. I suspect that would remove issues of different search_paths.

A really stupid question, in the cached query-string, wouldn't it be
possible to add the env specifics? Ie the string to check against is
something like

search_paths=...\n
SELECT ...

Or would there be too much stuff to append/prepend?

Best regards,
Marcus

On Wed, Nov 16, 2005 at 10:19:21AM +0100, Marcus Engene wrote:
> Martijn van Oosterhout wrote:
> >>This sql cache I think is a really good thing. Is there a reason
> >>Postgres hasn't got it? Would it be very hard to implement? From
> >>a naive perspective; make a hashvalue from the sql-string to
> >>quickly find the cached one, a "last used"-list for keeping
> >>track of which to delete when cache full etc seems close to
> >>trivial. Does the architecture/internal flow make it hard
> >>actually reuse the query data structure?
> >
> >It's hard to reuse the structure. Also, things like search_path mean
> >that the same query text can mean completely different things in
> >different backends. Most of the time it's planning that dominates, not
> >parsing so storing just the parser output seems somewhat useless.
> 
> Of course I didn't mean only the parse was to be saved. The planning
> goes there too.

It might be more useful to look at caching only planning and not
parsing. I'm not familiar with the output of the parsing stage, but
perhaps that could be hashed to use as a lookup into a cache of planned
queries. I suspect that would remove issues of different search_paths.
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461

On Mon, Nov 21, 2005 at 09:14:33PM +0100, Marcus Engene wrote:
> Jim C. Nasby wrote:
> >It might be more useful to look at caching only planning and not
> >parsing. I'm not familiar with the output of the parsing stage, but
> >perhaps that could be hashed to use as a lookup into a cache of planned
> >queries. I suspect that would remove issues of different search_paths.
> 
> A really stupid question, in the cached query-string, wouldn't it be
> possible to add the env specifics? Ie the string to check against is
> something like
> 
> search_paths=...\n
> SELECT ...
> 
> Or would there be too much stuff to append/prepend?

It's probably possible, but the thing is, afaik parsing just isn't a
bottleneck, so it's just not worth messing with that phase. If you do
end up with some super-complex query that does have a non-trivial parse
time, I believe views store a pre-parsed representation of the view
definition (otherwise there'd be issues with changing search_path
between when you create a view and when you use it), so you could
effectively cache something by just stuffing it into a view.
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461