Tuning queries on large database

Hi,

I have some problem of performance on a PG database, and I don't
know how to improve. I Have two questions : one about the storage
of data, one about tuning queries. If possible !

My job is to compare Oracle and Postgres. All our operational databases
have been running under Oracle for about fifteen years. Now I try to replace
Oracle by Postgres.

I have a test platform under linux (Dell server, 4 Gb RAM, bi-processor,
Linux Red Hat 9 (2.4.20-31.9)) with 2 databases, 1 with Oracle
(V8i or V9i it's quite the same), 1 with PG (7.4.2). Both databases
have the same structure, same content, about 100 Gb each. I developped
some benches, representative of our use of databases. My problem
is that I have tables (relations) with more than 100 millions rows,
and each row has about 160 fields and an average size 256 bytes.

For Oracle I have a SGA size of 500 Mb.
For PG I have a postgresql.conf as :
        max_connections = 1500
        shared_buffers = 30000
        sort_mem = 50000
        effective_cache_size = 200000
and default value for other parameters.

I have a table named "data" which looks like this :
bench=> \d data
                 Table "public.data"
   Column   |            Type             | Modifiers 
------------+-----------------------------+-----------
 num_poste  | numeric(9,0)                | not null
 dat        | timestamp without time zone | not null
 datrecu    | timestamp without time zone | not null
 rr1        | numeric(5,1)                | 
 qrr1       | numeric(2,0)                |       ...
 ... all numeric fields
 ...
 Indexes:
    "pk_data" primary key, btree (num_poste, dat)
    "i_data_dat" btree (dat)

It contains 1000 different values of "num_poste" and for each one 
125000 different values of "dat" (1 row per hour, 15 years). 

I run a vacuum analyze of the table.

bench=> select * from tailledb ;
 schema | relfilenode |      table       |   index    |  reltuples  |   size   
--------+-------------+------------------+------------+-------------+----------
 public |   125615917 | data             |            | 1.25113e+08 | 72312040
 public |   251139049 | data             | i_data_dat | 1.25113e+08 |  2744400
 public |   250870177 | data             | pk_data    | 1.25113e+08 |  4395480

My first remark is that the table takes a lot of place on disk, about
70 Gb, instead of 35 Gb with oracle.
125 000 000 rows x 256 b = about 32 Gb. This calculation gives an idea
not so bad for oracle. What about for PG ? How data is stored ?


The different queries of the bench are "simple" queries (no join,
sub-query, ...) and are using indexes (I "explained" each one to
be sure) :
Q1 select_court : access to about 700 rows  : 1 "num_poste" and 1 month
        (using PK : num_poste=p1  and dat between p2 and p3)
Q2 select_moy   : access to about 7000 rows : 10 "num_poste" and 1 month
        (using PK : num_poste between p1 and p1+10 and dat between p2 and p3)
Q3 select_long  : about 250 000 rows        : 2 "num_poste" 
        (using PK : num_poste in (p1,p1+2))
Q4 select_tres_long : about 3 millions rows : 25 "num_poste" 
        (using PK : num_poste between p1 and p1 + 25)

The result is that for "short queries" (Q1 and Q2) it runs in a few
seconds on both Oracle and PG. The difference becomes important with
Q3 : 8 seconds with oracle
     80 sec with PG
and too much with Q4 : 28s with oracle
                       17m20s with PG !

Of course when I run 100 or 1000 parallel queries such as Q3 or Q4, 
it becomes a disaster !
I can't understand these results. The way to execute queries is the
same I think. I've read recommended articles on the PG site.
I tried with a table containing 30 millions rows, results are similar.

What can I do ?

Thanks for your help !                     

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      joining column's datatypes do not match