On Oct 25, 2011, at 7:12 AM, alan wrote:
> I'm new to postgres and was wondering how to use EXPLAIN ANALYZE ....
>
> Can I use the output from ANALYZE EXPLAIN to estimate or predict the
> actual time
> it would take for a given query to return?
>
> I ask because I'm writing a typical web app that allows the user to
> build and submit a query
> to my DB. Since I don't know how "simple" or "complex" the user-
> generated queries will be
> I thought it might be possible to use the ANALYZE EXPLAIN output to
> make a "guestimation"
> about the expected return time of the query.
>
> I'd like to use this in my web-app to determine whether to run the
> query in real-time (user waits
> for results) or queue up the query (and notify the user once the query
> is finished). E.g.:
> if (the Total runtime" reported by explain analyze is > n ms) {
> tell the user that his request was submitted for processing, and
> notify the user once resuilts are available
> } else {
> run the query and wait for the results in real time.
> }
>
> Thanks,
> Alan
Check out the docs on EXPLAIN, http://www.postgresql.org/docs/9.0/static/sql-explain.html
EXPLAIN will give you an estimation. How well the internal statistics match the actual data will determine how close
theestimation is.
EXPLAIN ANALYZE will give you an estimation and the actuals. WARNING: EXPLAIN ANALYZE carries out the query. If there
anydata modifications in the query (UPDATE, INSERT, etc.) they will be carried out.
Note: ANALYZE is a different command and EXPLAIN is not a valid option for it. You'll be researching this command if
theestimates are way off.
Brent.