pgsql: HashAgg: before spilling tuples, set unneeded columns to NULL.

HashAgg: before spilling tuples, set unneeded columns to NULL.

This is a replacement for 4cad2534. Instead of projecting all tuples
going into a HashAgg, only remove unnecessary attributes when actually
spilling. This avoids the regression for the in-memory case.

Discussion: https://postgr.es/m/a2fb7dfeb4f50aa0a123e42151ee3013933cb802.camel%40j-davis.com
Backpatch-through: 13

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d8a7ce245095e3a70a2ad738c17be95593f68996

Modified Files
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src/backend/executor/nodeAgg.c | 123 ++++++++++++++++++++++++++++++-----------
src/include/nodes/execnodes.h  |   8 ++-
2 files changed, 96 insertions(+), 35 deletions(-)