pgsql: Make rewriter prevent auto-updates on views with conditional INS

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pgsql: Make rewriter prevent auto-updates on views with conditional INS

Dean Rasheed-3
Make rewriter prevent auto-updates on views with conditional INSTEAD rules.

A view with conditional INSTEAD rules and no unconditional INSTEAD
rules or INSTEAD OF triggers is not auto-updatable. Previously we
relied on a check in the executor to catch this, but that's
problematic since the planner may fail to properly handle such a query
and thus return a particularly unhelpful error to the user, before
reaching the executor check.

Instead, trap this in the rewriter and report the correct error there.
Doing so also allows us to include more useful error detail than the
executor check can provide. This doesn't change the existing behaviour
of updatable views; it merely ensures that useful error messages are
reported when a view isn't updatable.

Per report from Pengzhou Tang, though not adopting that suggested fix.
Back-patch to all supported branches.

Discussion: https://postgr.es/m/CAG4reAQn+4xB6xHJqWdtE0ve_WqJkdyCV4P=trYr4Kn8_3_PEA@...

Branch
------
REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fd5476b79c2ff63dd32e154198f8201553180e58

Modified Files
--------------
src/backend/executor/execMain.c               |  8 ++--
src/backend/rewrite/rewriteHandler.c          | 60 ++++++++++++++++++++++++---
src/test/regress/expected/updatable_views.out | 21 ++++++++++
src/test/regress/sql/updatable_views.sql      | 14 +++++++
4 files changed, 94 insertions(+), 9 deletions(-)