pgsql: Change various deparsing functions to return NULL for invalid in

Change various deparsing functions to return NULL for invalid input.

Previously, some functions returned various fixed strings and others
failed with a cache lookup error. Per discussion, standardize on
returning NULL. Although user-exposed "cache lookup failed" error
messages might normally qualify for bug-fix treatment, no back-patch;
the risk of breaking user code which is accustomed to the current
behavior seems too high.

Michael Paquier

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/976b24fb477464907737d28cdf18e202fa3b1a5b

Modified Files
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src/backend/utils/adt/ruleutils.c | 183 ++++++++++++++++++++++++++++--------
src/test/regress/expected/rules.out | 37 ++++++++
src/test/regress/sql/rules.sql | 8 ++
3 files changed, 190 insertions(+), 38 deletions(-)