Обсуждение: performance tuning queries

Hi All;

I'm looking for tips / ideas per performance tuning some specific queries.
These are generally large tables on a highly active OLTP system
(100,000 - 200,000 plus queries per day)

First off, any thoughts per tuning inserts into large tables. I have a large
table with an insert like this:

insert into public.bigtab1 (text_col1, text_col2, id) values ...

                QUERY PLAN
------------------------------------------
 Result  (cost=0.00..0.01 rows=1 width=0)
(1 row)

The query cost is low but this is one of the slowest statements per pgfouine







Next we have a select count(*) that  also one of the top offenders:

select count(*) from public.tab3  where user_id=31
and state='A'
and amount>0;

                                             QUERY PLAN
-----------------------------------------------------------------------------------------------------
 Aggregate  (cost=3836.53..3836.54 rows=1 width=0)
   ->  Index Scan using order_user_indx ontab3 user_id  (cost=0.00..3834.29
rows=897 width=0)
         Index Cond: (idx_user_id = 31406948::numeric)
         Filter: ((state = 'A'::bpchar) AND (amount > 0::numeric))
(4 rows)

We have an index on the user_id but not on the state or amount,

add index to amount ?



Thoughts ?

> First off, any thoughts per tuning inserts into large tables. I have a
> large
> table with an insert like this:
>
> insert into public.bigtab1 (text_col1, text_col2, id) values ...
>
>                 QUERY PLAN
> ------------------------------------------
>  Result  (cost=0.00..0.01 rows=1 width=0)
> (1 row)
>
> The query cost is low but this is one of the slowest statements per
> pgfouine

    Possible Causes of slow inserts :

    - slow triggers ?
    - slow foreign key checks ? (missing index on referenced table ?)
    - functional index on a slow function ?
    - crummy hardware (5 MB/s RAID cards, etc)
    - too many indexes ?

> Next we have a select count(*) that  also one of the top offenders:
>
> select count(*) from public.tab3  where user_id=31
> and state='A'
> and amount>0;
>
>                                              QUERY PLAN
> -----------------------------------------------------------------------------------------------------
>  Aggregate  (cost=3836.53..3836.54 rows=1 width=0)
>    ->  Index Scan using order_user_indx ontab3 user_id
> (cost=0.00..3834.29
> rows=897 width=0)
>          Index Cond: (idx_user_id = 31406948::numeric)
>          Filter: ((state = 'A'::bpchar) AND (amount > 0::numeric))
> (4 rows)
>
> We have an index on the user_id but not on the state or amount,
>
> add index to amount ?

    Can we see EXPLAIN ANALYZE ?

    In this case the ideal index would be multicolumn (user_id, state) or
(user_id,amount) or (user_id,state,amount) but choosing between the 3
depends on your data...

    You could do :

SELECT count(*), state, amount>0  FROM public.tab3  where user_id=31 GROUP
BY state, amount>0;

    And post the results.

am  Wed, dem 26.11.2008, um 21:21:04 -0700 mailte Kevin Kempter folgendes:
> Next we have a select count(*) that  also one of the top offenders:
>
> select count(*) from public.tab3  where user_id=31
> and state='A'
> and amount>0;
>
>                                              QUERY PLAN
> -----------------------------------------------------------------------------------------------------
>  Aggregate  (cost=3836.53..3836.54 rows=1 width=0)
>    ->  Index Scan using order_user_indx ontab3 user_id  (cost=0.00..3834.29
> rows=897 width=0)
>          Index Cond: (idx_user_id = 31406948::numeric)
>          Filter: ((state = 'A'::bpchar) AND (amount > 0::numeric))
> (4 rows)
>
> We have an index on the user_id but not on the state or amount,
>
> add index to amount ?

Depends.

- Is the index on user_id a unique index?
- how many different values are in the table for state, i.e., maybe an
  index on state can help
- how many rows in the table with amount > 0? If almost all rows
  contains an amount > 0 an index can't help in this case


Andreas
--
Andreas Kretschmer
Kontakt:  Heynitz: 035242/47150,   D1: 0160/7141639 (mehr: -> Header)
GnuPG-ID:   0x3FFF606C, privat 0x7F4584DA   http://wwwkeys.de.pgp.net

Kevin Kempter schrieb:
> Hi All;
>
> I'm looking for tips / ideas per performance tuning some specific queries.
> These are generally large tables on a highly active OLTP system
> (100,000 - 200,000 plus queries per day)
>
> First off, any thoughts per tuning inserts into large tables. I have a large
> table with an insert like this:
>
> insert into public.bigtab1 (text_col1, text_col2, id) values ...
>
>                 QUERY PLAN
> ------------------------------------------
>  Result  (cost=0.00..0.01 rows=1 width=0)
> (1 row)
>
> The query cost is low but this is one of the slowest statements per pgfouine
>
Do you insert multiple values in one transaction, or one transaction per
insert?