Re: hash joins and pga/temp space?
- From: ryan_gaffuri@xxxxxxxxxxx
- To: peters@xxxxxxxxx, _oracle_L_list <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 14 Nov 2006 18:54:16 +0000
I am really just looking for a way to predictively forecast how much hash space
I need for a given query. Assuming the plan is the same. So as I add data I can
know in advance what I will need instead of a try and find out.
The system is in development. I don't have production data. So we are testing
with smaller data sets.
-------------- Original message --------------
From: Peter Sylvester <peters@xxxxxxxxx>
> I don't have all the answers, but I did run into some issues with hash
> joins degenerating into nested-loop joins when the record count
> increased. I ended up cranking up the PGA_AGGREGATE_TARGET since I am
> doing the auto-PGA thing (10.2.0.2/Solaris10), and it seemed to solve my
> problem. I also added USE_HASH hints, but I *think* it will still do
> something else if it does not have enough memory for the hash table.
> Unfortunately I do not have a representative test system for this
> warehouse style implementation.
>
> Note that if you use the USE_HASH hint the smaller table should be the
> first parameter.
>
> I also recall reading that there are some upper limits for the hash_area
> and sort_area memory sizes (200mb?) when using the auto-PGA, and some
> undocumented parameters may be required for getting larger sizes. In my
> case I was able to get away with adjusting PGA_AGGREGATE_TARGET up so
> did not get into that.
>
> There is some interested reading on hash joins (as well as sorting
> costs, and lots of other things...) in Jonathan Lewis' book "Cost-Based
> Oracle Fundamentals", which you may want to check out.
>
> --Peter
>
> --
> //www.freelists.org/webpage/oracle-l
>
>
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