pgsql: Extend ExecBuildAggTrans() to support a NULL pointer check.

Extend ExecBuildAggTrans() to support a NULL pointer check.

Optionally push a step to check for a NULL pointer to the pergroup
state.

This will be important for disk-based hash aggregation in combination
with grouping sets. When memory limits are reached, a given tuple may
find its per-group state for some grouping sets but not others. For
the former, it advances the per-group state as normal; for the latter,
it skips evaluation and the calling code will have to spill the tuple
and reprocess it in a later batch.

Add the NULL check as a separate expression step because in some
common cases it's not needed.

Discussion: https://postgr.es/m/20200221202212.ssb2qpmdgrnx52sj%40alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c954d49046504bde0a80b5fec53f4321dd88f1ea

Modified Files
--------------
src/backend/executor/execExpr.c       | 39 ++++++++++++++++++++++++++++++-----
src/backend/executor/execExprInterp.c | 17 +++++++++++++++
src/backend/executor/nodeAgg.c        |  3 ++-
src/backend/jit/llvm/llvmjit_expr.c   | 39 +++++++++++++++++++++++++++++++++++
src/include/executor/execExpr.h       |  8 +++++++
src/include/executor/executor.h       |  2 +-
6 files changed, 101 insertions(+), 7 deletions(-)