I'm by no means an expert with Oracle, but I know enough in terms of how to analyze explain plans and perform basic optimizations. However, this time, Oracle has stumped me with it's explain plan.
Basically, I've got a single table that is very large (27 million rows) with an index on ScheduleID. What I'm trying to do is get all the records that have a ScheduleID that match one of the ScheduleIDs in a given list (currently stored in a temp table). The temp table will probably have around 100 rows, although it could be up to 500 rows. And for each row in temp row, it will have up to around 10 matching records in the large table. So we are talking about the query returning about 5000 rows max (out of 27 million).
Now, for some reason, Oracle thinks that the best solution is to use a hash join and do a full table scan of the large table. You can see the query and its explain plan that Oracle generates below.
select scheduleintervalid, io_scheduleinterval.version
inner join io_querytemp tmp on tmp.data = scheduleid
This doesn't seem to make any sense to me. It is not taking advantage of the index on ScheduleID, which I would think would make the query significantly faster. Now from my understanding, the hash join couldn't take advantage of the index, but wouldn't a nested loop join be much faster. Am I missing something here?
Is there something else I can do to speed up the query (I've tried things like using a dynamic IN clause)?
Thanks for the help!