Re: Question about SQL performance

On Mon, Jun 04, 2007 at 11:18:30PM -0400, 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?
>
> SELECT * FROM mytable WHERE item = 5;
> SELECT * FROM mytable WHERE item = 10;
>
> Obviously to me or you they could use the same plan. 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.

Although they could use the same plan, it is possible that using the
same plan is non-optimal. For example, if I know that 99% of the table
contains item = 5, but only 1% of the table contains item = 10, then
the 'best plan' may be a sequential scan for item = 5, but an index scan
for item = 10.

In the case of a prepared query, PostgreSQL will pick a plan that will
be good for all values, which may not be best for specific queries. You
save parsing time and planning time, but may risk increasing execution
time.

> Also, what's the difference between prepared statements (using
> PREPARE and EXECUTE) and regular functions (CREATE FUNCTION)? How do
> they impact performance? From what I understand there is no exact
> parallel to stored procedures (as in MS SQL or oracle, that are
> completely precompiled) in Postgresql. 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 think you can find all of these answers in the documentation, including
my comments about prepared queries. Does it matter if the program is
precompiled? I believe it is, but why would it matter?

Are you addressing a real performance problem? Or are you trying to avoid
issues that you are not sure if they exist or not? :-)

Prepared queries are going to improve performance due to being able to
execute multiple queries without communicating back to the
client. Especially for short queries, network latency can be a
significant factor for execution speed.

Cheers,
mark

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