pgsql: Rewrite recurse_union_children to iterate, rather than recurse.

Rewrite recurse_union_children to iterate, rather than recurse.

Also, rename it to plan_union_chidren, so the old name wasn't
very descriptive.  This results in a small net reduction in code,
seems at least to me to be easier to understand, and saves
space on the process stack.

Patch by me, reviewed and tested by Ashutosh Bapat and Rajkumar
Raghuwanshi.

Discussion: http://postgr.es/m/CA+TgmoaLRAOqHmMZx=ESM3VDEPceg+-XXZsRXQ8GtFJO_zbMSw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/49525c46309828b3024fe8040fa99c7dcc83933d

Modified Files
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src/backend/optimizer/prep/prepunion.c | 100 ++++++++++++++++-----------------
1 file changed, 47 insertions(+), 53 deletions(-)