I would like to ask again, because I feel really stupid, I get no answer. I always got an answer to my
questions when I mailed in mailing lists until now. What's happening? At least somebody could answer
simply: "Your question is not worth an answer!" but no answer makes me feel really stupid. As I have
spent a lot of time reading documentation and making tests (perhaps not enough) I would really be
pleased if I had an answer to this simple problem (tell me if it is my fault please):
Original message from: Dragos Stoichita
> Hi, I'm a new user to SQL and PostgreSQL so perhaps my questions below will be a little stupid so
>please excuse me.
>
> I do this:
>
> CREATE TABLE t1 ( PRIMARY KEY (f1), f1 INTEGER, f2 INTEGER);
> CREATE TABLE t2 ( PRIMARY KEY (f1), f1 INTEGER, f2 INTEGER);
>
> Then I fill each of these tables with say, around 10000 rows.
>
> When I do:
>
> SELECT f2 FROM t1 WHERE f1 > 100;
>
> It is amazingly fast! It takes less than 1 second. And it returns around 3000 rows.
>
> I do then:
>
> SELECT f2 FROM t2 WHERE f1 > 100;
>
> It is also amazingly fast and returns around 4000 rows.
>
> Then I do:
>
> SELECT f2 FROM t1 WHERE f1 > 100 INTERSECT SELECT f2 FROM t2 WHERE f1 > 100;
>
> And it is incredibly *SLOW*!!! I really don't understand, I run postmaster on a 400Mhz pc with 64 megs
>of ram. What's happening? It is only an intersection of integers. If I had to do it in C, I would Quicksort
>the results from the first query, Quicksort the results from the second query, then unique them, then
>intersect them. On a 400 Mhz processor I think it would take less than 1 second. I tested my Quicksort
>routines on a Pentium 120 and remembered it sorted more than 100000 integers per second. And a
>unique algorithm when the elements are ordered is very fast. The same for an intersection algorithm. But
>it takes more than 8 seconds for PostgreSQL to process the INTERSECT.
>
> Is there an explanation? Is it my fault? Please help me I already switched from another database to this
>one and hoped PostgreSQL would perform well :(
>
> Dragos Stoichita, 19 year old student in electronics at ESIEE (http://www.esiee.fr)
>
>
>
>