I have a question about Database management

[nineday <nine9day@xxxxxxxxx>]

I have a trouble.
My DBMS is PostgreSQL which is accessory to Red hat 9.0.

At the initial stages, it was very quick when select ,but operated for several months , it  become to very slow ,especially in a pivotal table.
So,I exported data ,and recreate tables , finally import data, it get right .

 

I do not know why. and what is the solution to my trouble 

                    Thank you very much ~~