Re: Question about SQL performance

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