Index not used,

Hi!<br />I've have this table:<br /><br />TABLE BB : There isn't any primary key, and is it more or less order  (I
mean,tt always is increased in each row, and id is nearly ordered)<br />ex:<br /><br />id, op, atr, tt<br /><br /> 1  
0   X,   1 <br />2   0    A   3<br />3   0   X    5<br />..........<br />1   0   X   51<br />.......<br />85  1    l  
150<br/>86  2   po 155<br />2    0   X   178<br />87  3   1   189<br />....<br /><br />I VACUUM ANALYZE each 10.000
insertsmore or less<br />in my case op only can have 3 values;<br />I've created an index on (id,op,tt) to improve the
nextquery, that is executed very often:<br />"SELECT * FROM BB WHERE id="+ id+" AND op=0 order by tt desc;";<br
/>(becausethe only row I need is the one that has the highest tt)<br /><br />but, after an hour running the program
(morethan 90.000 rows), I stopped it and <br />"EXPAIN SELECT * FROM BB WHERE id="+ id+" AND op=0 order by tempst
desc;";<br/>But, my sorprise was that the index wasn't be used. Always do a Seq Scan.<br /><br />Have I to define the
indexin other columns? Have I to VACUUM ANALYZE more often? <br /><br />There is other option to avoid the sort that is
donebecause of ORDER BY?<br /><br />Thanks a lot<br /><br />yours,<br /><br />Cris..