Question

I am trying to understand the performance of a query that I ve written in Oracle. At this time I only have access to SQLDeveloper and its execution timer. I can run SHOW PLAN but cannot use the auto trace function.

The query that I ve written runs in about 1.8 seconds when I press "execute query" (F9) in SQLDeveloper. I know that this is only fetching the first fifty rows by default, but can I at least be certain that the 1.8 seconds encompasses the total execution time plus the time to deliver the first 50 rows to my client?

When I wrap this query in a stored procedure (returning the results via an OUT REF CURSOR) and try to use it from an external application (SQL Server Reporting Services), the query takes over one minute to run. I get similar performance when I press "run script" (F5) in SQLDeveloper. It seems that the difference here is that in these two scenarios, Oracle has to transmit all of the rows back rather than the first 50. This leads me to believe that there is some network connectivity issues between the client PC and Oracle instance.

My query only returns about 8000 rows so this performance is surprising. To try to prove my theory above about the latency, I ran some code like this in SQLDeveloper:

declare
    tmp sys_refcursor;
begin
   my_proc(null, null, null, tmp);
end;

...And this runs in about two seconds. Again, does SQLDeveloper s execution clock accurately indicate the execution time of the query? Or am I missing something and is it possible that it is in fact my query which needs tuning?

Can anybody please offer me any insight on this based on the limited tools I have available? Or should I try to involve the DBA to do some further analysis?

Answer 1

"I know that this is only fetching the first fifty rows by default, but can I at least be certain that the 1.8 seconds encompasses the total execution time plus the time to deliver the first 50 rows to my client?"

No, it is the time to return the first 50 rows. It doesn t necessarily require that the database has determined the entire result set.

Think about the table as an encyclopedia. If you want a list of animals with names beginning with A or Z , you ll probably get Aardvarks and Alligators pretty quickly. It will take much longer to get Zebras as you d have to read the entire book. If your query is doing a full table scan, it won t complete until it has read the entire table (or book), even if there is nothing to be picked up in anything after the first chapter (because it doesn t know there isn t anything important in there until it has read it).

declare
    tmp sys_refcursor;
begin
   my_proc(null, null, null, tmp);
end;

This piece of code does nothing. More specifically, it will parse the query to determine that the necessary tables, columns and privileges are in place. It will not actually execute the query or determine whether any rows meet the filter criteria.

If the query only returns 8000 rows it is unlikely that the network is a significant problem (unless they are very big rows).

Ask your DBA for a quick tutorial in performance tuning.

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