Re: Performance Tuning Large PL/PGSQL Stored Procedure - Mailing list pgsql-performance

On Tue, Mar 23, 2010 at 5:00 PM, Eliot Gable
<egable+pgsql-performance@gmail.com> wrote:
> The complex type contains roughly 25 fields, mostly text, plus another 10
> REFCURSORs.

How many rows min/max/avg are coming back in your refcursors?  Are you
using cursors in order to return multiple complex data structures
(sets, etc) in a single function call?

> The application that calls the stored procedure was also written by me in
> C++ and uses asynchronous libpq API commands to execute a single SQL
> transaction which calls the stored procedure and also performs a FETCH ALL
> on all open cursors. It then returns all results into various structures.
> All rows of all cursors that are open are always used for every call to the
> stored procedure.
>
> The stored procedure implements various logic which determines which tables
> in the database to query and how to filter the results from those queries to
> return only the relevant information needed by the C++ application.
>
> Currently, in terms of optimization, I have taken the following approaches
> based on the following reasoning:
>
> 1. For all queries whose results need to return to the C++ application, I
> utilize cursors so that all results can be readied and generated by the
> stored procedure with just one call to the PostgreSQL backend. I accomplish
> this using asynchronous libpq API calls to issue a single transaction to the
> server. The first command after the BEGIN is a SELECT * FROM
> MyStoredProc(blah), which is then followed by FETCH ALL commands for each
> cursor that the stored procedure leaves open. I then follow up with multiple
> API calls to return the results and retrieve the rows from those results.
> This minimizes the amount of back-and-forth between my C++ application and
> the database backend.
>
> 1a. Incidentally, I am also using cursors for most queries inside the stored
> procedure that do not return results to the C++ application. I am unsure
> whether this incurs a performance penalty compared to doing, for example, a
> SELECT ... INTO (var1, var2, ...) within the body of the stored procedure.
> Instead of using SELECT ... INTO, I am using OPEN cursor_name; FETCH
> cursor_name INTO (var1, var2).
>
> 2. I have built indexes on all columns that are used in where clauses and
> joins.
>
> 3. I use lots of joins to pull in data from various tables (there are around
> 60 tables that are queried with each call to the stored procedure).
>
> 4. When performing joins, the first table listed is the one that returns the
> most context-specific results, which always also means that it has the
> most-specific and fewest number of relevant rows. I then join them in order
> of least number of result rows with all inner joins preceding left outer
> joins.
>
> 5. Instead of using UNIONs and EXCEPT clauses, I use multiple WITH clauses
> to define several different query-specific views. I order them such that I
> can join additional tables in later WITH clauses to the views created

WITH clauses can make your queries much easier to read and yield great
speedups if you need to access the table expression multiple times
from other parts of the query.  however, in some cases you can get
into trouble because a standard set of joins is going to give the
planner the most flexibility in terms of query optimization.

> previously in a way that minimizes the number of rows involved in the JOIN
> operations while still providing provably accurate result sets. The EXCEPT
> clauses are then replaced by also defining one view which contains a set of
> IDs that I want filtered from the final result set and using a WHERE id NOT
> IN (SELECT id FROM filtered_view). Typically, this approach leaves me with
> just one UNION of two previously defined views (the union is required


UNION is always an optimization target (did you mean UNION ALL?)

> 7. When I have a query I need to execute whose results will be used in
> several other queries, I currently open the cursor for that query using the
> FOR ... LOOP construct to retrieve all records in the result set and build a
> result array using the array_append() method. I then do an unnest(my_array)

do not use array_append.  always do array(select ...) whenever it is
possible. when it isn't, rethink your problem until it is possible.
only exception is to use array_agg aggregate if your problem really is
an aggregation type of thing.  as a matter of fact, any for...loop is
an optimization target because a re-think will probably yield a query
that does the same thing without paying for the loop.

>
> For most of the joins, they simply join on foreign key IDs and no additional
> filtering criteria are used on their information. Only a handful of the
> joined tables bring in additional criteria by which to filter the result
> set.
>
> The approach used in 7 with cursors and building a result array which is
> then unnested has me worried in terms of performance. It seems to me there
> should be some better way to accomplish the same thing.
>
> The stored procedure does not perform updates or inserts, only selects.
>
>
> Anyway, if anyone has some insights into performance tweaks or new
> approaches I might try that may lead to enhanced performance, I would
> greatly appreciate hearing about them. I am not completely dissatisfied with
> the performance of the stored procedure, but this is going to be used in a
> very high volume environment (hundreds or possibly even thousands of calls
> to this stored procedure every second). The more performant it is, the less
> hardware I need to deploy. It currently takes about 45ms to execute the
> query and retrieve all of the results into the C++ application. Query
> execution time takes up about 16ms of that 45ms. This is on a 3-year old
> Core 2 Duo, so it's not exactly top-of-the-line hardware.

If you are chasing milliseconds, using C/C++, and dealing with complex
data structures coming in/out of the database, I would absolutely
advise you to check out the libpqtypes library (disclaimer, I co-wrote
it!) in order to speed up data transfer. The library is highly
optimized and facilitates all transfers in binary which yields good
gains when sending types which are expensive to hammer to text (bytea,
timestamp, etc).

In addition, using libpqtypes you can use arrays of composites (in
8.3+) to send/receive complex structures (even trees, etc) and pull
the entire set of data in a single query.  This is an alternative to
the refcursor/fetch method which involves extra round trips and has
other problems (but is the way to go if you need to progressive fetch
large amounts of data).

As a general tip, I suggest 'divide and conquer'.  Sprinkle your
procedure with 'raise notice %', gettimeofday(); And record the time
spent on the various steps of the execution.  This will give better
measurements then pulling pieces of the function out and running them
outside with constants for the arguments.  Identify the problem spots
and direct your energies there.

merlin

http://libpqtypes.esilo.com/
http://pgfoundry.org/projects/libpqtypes/