Re: Help request to improve function performance

2009/4/22 sarikan <serefarikan@kurumsalteknoloji.com>

Dear members of the list,
I have a function which returns a custom type, that has only two fields,
each of them being varchar arrays.
The reason that I have written this function is that I have a table
basically with the following structure (with simplified column names)

name_col1  name_col2 sessionId
value1       value3       id1
value2       value2       id1
value4       value4       id1
value7       value4       id2
value2       value2       id2
value4       value4       id2
value1       value5       id3

Why not post your REAL schema? It would make life easier, both for you and for people trying to help.

 

So mutliple rows are bound together with sessionIds, and I need to get back
all rows with a query, grouped by sessionID. However, group by sql statement
does not solve my problem, since I get back a lot of rows, which I have to
group into objects again in my application. What I need is a way to return
all rows having the same sessionId as a single row. Of course this is not
possible with this table, so I've created a custom type, which has array
type columns. The following function gets all rows that belongs to a
patient, and for each session id, it inserts rows with that session id into
array fields of the custom type.
The problem is, it is very slow! Getting back all the rows with a select
takes 360 ms, while getting back the results of this function takes 50
seconds! Is there any way I can make the following function faster, or any
other methods you can recommend to do what I'm trying to do?  I am trying to
avoid hundreds of calls to db, or grouping query results in my middleware
application. Here comes the function, and your help will be much
appreciated.

Best Regards
Seref

(below code edited to be more readable; logic unchanged)
 

CREATE or REPLACE FUNCTION getNodeContainers( context_Id varchar) RETURNS
setof NodesContainer AS
$$
DECLARE
archetype_data_row app.archetype_data;
archetype_data_row_main app.archetype_data;
nodescontainervar NodesContainer;
session_Id varchar;
indexVar integer := 0;
BEGIN
   CREATE TEMP TABLE all_rows_of_patient AS select * from app.archetype_data
   WHERE context_id = context_Id;
   FOR session_Id IN
     SELECT distinct session_id from all_rows_of_patient
   LOOP -- do the following for each session_ID
       indexVar := 0;
       FOR archetype_data_row IN --select rows that belong to this session ID
               SELECT * from all_rows_of_patient
       WHERE session_id = session_Id and context_id = context_Id
       LOOP
               nodescontainervar.name[indexVar] := archetype_data_row.name;
               nodescontainervar.context_Id[indexVar] := archetype_data_row.context_Id;
               indexVar := indexVar + 1;
       END LOOP;
       return NEXT nodescontainervar;
   END LOOP;
   drop table all_rows_of_patient;
   return;
END;
$$ LANGUAGE 'plpgsql';

please read above code - thats what postgres actually executes. column names have precedence before variable names in name resolution.
conditions like
 WHERE context_id = context_Id;
 WHERE session_id = session_Id and context_id = context_Id
are obviously no-op conditions, not what you really want.

I hope now it's clear now why this function has long execution time :)


some other remarks:

1) you use temp tables inside a function, which is rather bad (search archives for explanation). try to avoid it.
2) usage of indexvar is not needed - there are array operators and functions
3) if you get rid of temp table, this function could be marked as STABLE,which will prevent penalty in some strange situations
4) mark your function as STRICT, which will save some CPU cycles when someone calls it on null input

HTH.
 

--
Filip Rembiałkowski
JID,mailto:filip.rembialkowski@gmail.com
http://filip.rembialkowski.net/