[firebird-support] SQL Performance question

[Michael Vilhelmsen]

Hi

I have a table containing some 33.879.139 records.

If I do a simple select like this:


Select Dato from Transaktioner
Where
  VareFrvStrNr='090179'
  AND Art=11
  And Transaktioner.Afdeling_ID<>'99999'


it fecthes 11 records within 0.3 seconds.
It will use the index on VareFrvStrNr according to the plan.
PLAN (TRANSAKTIONER INDEX (TRANS_VAREFRVSTRNR))



If I instead uses this SQL


Select Min(Dato)As MinimuSTG from Transaktioner
Where
  VareFrvStrNr='090179'
  AND Art=11
  And Transaktioner.Afdeling_ID<>'99999'

It takes at least 3 seconds. Often more. 
Now the plan is like this:
PLAN (TRANSAKTIONER ORDER TRANS_DATO INDEX (TRANS_VAREFRVSTRNR))


The TRANS_DATO index has almost no duplicates.
The TRANS_VAREFRVSTRNR can have some dublicates.



Why is there such a big difference is the performance?
can I do anything to optimize this?

Michael