pgsql: Invent GENERIC_PLAN option for EXPLAIN.

Invent GENERIC_PLAN option for EXPLAIN.

This provides a very simple way to see the generic plan for a
parameterized query. Without this, it's necessary to define
a prepared statement and temporarily change plan_cache_mode,
which is a bit tedious.

One thing that's a bit of a hack perhaps is that we disable
execution-time partition pruning when the GENERIC_PLAN option
is given. That's because the pruning code may attempt to
fetch the value of one of the parameters, which would fail.

Laurenz Albe, reviewed by Julien Rouhaud, Christoph Berg,
Michel Pelletier, Jim Jones, and myself

Discussion: https://postgr.es/m/0a29b954b10b57f0d135fe12aa0909bd41883eb0.camel@cybertec.at

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3c05284d83b230728e59a25e828992037ef77096

Modified Files
--------------
doc/src/sgml/ref/explain.sgml | 70 +++++++++++++++++++++++++++++++----
src/backend/commands/explain.c | 11 ++++++
src/backend/executor/execMain.c | 2 +-
src/backend/executor/execPartition.c | 10 +++--
src/backend/parser/analyze.c | 29 +++++++++++++++
src/bin/psql/tab-complete.c | 4 +-
src/include/commands/explain.h | 1 +
src/include/executor/executor.h | 22 ++++++++---
src/test/regress/expected/explain.out | 42 +++++++++++++++++++++
src/test/regress/sql/explain.sql | 26 +++++++++++++
10 files changed, 197 insertions(+), 20 deletions(-)