pgsql: Fix planner to make a reasonable assumption about the amount of

Log Message:
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Fix planner to make a reasonable assumption about the amount of memory space
used by array_agg(), string_agg(), and similar aggregate functions that use
"internal" as their transition datatype.  The previous coding thought this
took *no* extra space, since "internal" is pass-by-value; but actually these
aggregates typically consume a great deal of space.  Per bug #5608 from
Itagaki Takahiro, and fix suggestion from Hitoshi Harada.

Back-patch to 8.4, where array_agg was introduced.

Tags:
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REL8_4_STABLE

Modified Files:
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    pgsql/src/backend/optimizer/util:
        clauses.c (r1.277.2.2 -> r1.277.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c?r1=1.277.2.2&r2=1.277.2.3)