Message-ID: <466523F0.5020306@archonet.com>
Date: Tue, 05 Jun 2007 09:50:56 +0100
From: Richard Huxton <dev@archonet.com>
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To: Jason Lustig <lustig@brandeis.edu>
Cc: pgsql-performance@postgresql.org
Subject: Re: Question about SQL performance
References: <F84FCD04-CB11-4CA2-838A-0ABA75A7F1A2@brandeis.edu>
In-Reply-To: <F84FCD04-CB11-4CA2-838A-0ABA75A7F1A2@brandeis.edu>
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Jason Lustig wrote:
> I have some questions about the performance of certain types of SQL 
> statements.
> 
> What sort of speed increase is there usually with binding parameters 
> (and thus preparing statements) v. straight sql with interpolated 
> variables? Will Postgresql realize that the following queries are 
> effectively the same (and thus re-use the query plan) or will it think 
> they are different?

PG will plan "raw" sql every time you issue a query.

>     SELECT * FROM mytable WHERE item = 5;
>     SELECT * FROM mytable WHERE item = 10;
> 
> Obviously to me or you they could use the same plan. 

Except that in-between query 1 and 2 I inserted 10 million rows where 
item=10. Still obvious?

 > From what I
> understand (correct me if I'm wrong), if you use parameter binding - 
> like "SELECT * FROM mytable WHERE item = ?" - Postgresql will know that 
> the queries can re-use the query plan, but I don't know if the system 
> will recognize this with above situation.

If you are using PREPARE/EXECUTE (or your client-side library is doing 
it for you).

> Also, what's the difference between prepared statements (using PREPARE 
> and EXECUTE) and regular functions (CREATE FUNCTION)? How do they impact 
> performance? 

Functions can be in any language, but if they both are in SQL and do the 
same thing, no real difference.

 > From what I understand there is no exact parallel to stored
> procedures (as in MS SQL or oracle, that are completely precompiled) in 
> Postgresql. 

You can write functions in C - that's compiled. Not sure if java 
procedural code has its byte-code cached between sessions.

 > At the same time, the documentation (and other sites as
> well, probably because they don't know what they're talking about when 
> it comes to databases) is vague because  PL/pgSQL is often said to be 
> able to write stored procedures but nowhere does it say that PL/pgSQL 
> programs are precompiled.

I don't see the connection.
1. You can write procedural code in pl/pgsql
2. It's not precompiled (it's "compiled" on first use)

Are you looking to solve a particular problem?

-- 
   Richard Huxton
   Archonet Ltd