pgsql: Improve performance of find_all_inheritors()

Improve performance of find_all_inheritors()

Previous coding uses three nested loops which obviously were a pain for
large number of table's children. Patch replaces inner loop with
a hashmap.

Author: Aleksander Alekseev
Reviewed-by: me

https://commitfest.postgresql.org/13/1058/

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/827d6f977940952ebef4bd21fb0f97be4e20c0c4

Modified Files
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src/backend/catalog/pg_inherits.c | 58 +++++++++++++++++++++++++++++----------
1 file changed, 44 insertions(+), 14 deletions(-)