Re: max 5% of pga_aggregate_target for a single serial session

Chapter 9, page 33 - Cat-Hash-strophes
in the seminar notes. (The page number
may have changed a little).

If you have a plan like this, all the leading tables have been hashed into memory at
the same time.

HASH JOIN
    TABLE ACCESS (FULL) OF TABLE_4
    HASH JOIN

        TABLE ACCESS (FULL) OF TABLE_3
        HASH JOIN
            TABLE ACCESS (FULL) OF TABLE_2
            TABLE ACCESS (FULL) OF TABLE_1

It is true that Oracle only works out the cost of joining two objects at a time - but the actual execution can pass results from stage to step before a step is completed, just like the NL join.

In this example, the 10053 trace showed (if I recall correctly):

    calc 1: join t2 to t1 with swap_join_inputs     calc2: join t3 to result1 with swap_join_inputs     calc3: join t4 to result2 with swap_join_inputs

Regards

Jonathan Lewis
http://www.jlcomp.demon.co.uk

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  who can answer the questions, but the
  person who can question the answers -- T. Schick Jr

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• Original Message ----- To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Wednesday, December 31, 2003 12:19 AM

> I've alway understood that joining occurs always in two steps, first two
> tables, then their result (row source) with next table and so on, so
there's
> no need for more than 2 hash tables for example?
>
> Tanel.
>

-- 
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Author: Jonathan Lewis
  INET: jonathan_at_jlcomp.demon.co.uk

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