Thank you BugsLaw for your interest.

I've checked the select for only one table (TASK) earlier. I had pretty good results - 100k rows for about 3 seconds. And I was very surprised that after joining it with LOCATION perfromance dropped so much.

Here are the results with index:

Starting transaction...
Preparing statement: SELECT
T.ID_TASK
FROM
TASK T
WHERE
T.IDSHEDULE IN (8169797)
Statement prepared (elapsed time: 3.386s).
Field #01: TASK.ID_TASK Alias:ID_TASK Type:INTEGER
PLAN (T INDEX (IDX_TASK_IDSHEDULE))


Executing statement...
Statement executed (elapsed time: 0.000s).
45747 fetches, 4 marks, 390 reads, 4 writes.
0 inserts, 0 updates, 0 deletes, 2704 index, 18924 seq.
Delta memory: 53744 bytes.
Total execution time: 3.760s
Script execution finished.

I have a copy of the database, but reactivating the index would take ages.

I've recomputed all statistics and result for IDX_TASK_IDSHEDULE is 0.000004.