pgsql: Report duplicate property and label names with a proper error

Report duplicate property and label names with a proper error

Adding a property with the same name multiple times to a label on an
element, either within a single PROPERTIES clause or across statements
via ALTER PROPERTY GRAPH ... ADD PROPERTIES, previously failed with a
unique-index violation on pg_propgraph_label_property. The same class
of bug existed for labels: listing the same label multiple times on
one element in CREATE PROPERTY GRAPH, or adding a label to an element
that already has it via ALTER PROPERTY GRAPH ... ADD LABEL, failed
with a unique-index violation on pg_propgraph_element_label. Detect
the duplicates up front and raise a friendlier error in both cases.

For properties, the cross-statement duplicate is caught by a syscache
probe before insertion. The in-clause duplicate could have been
caught by the same probe if we issued a CommandCounterIncrement()
between property inserts, but that forces a catalog invalidation per
property purely to detect a condition we can check for free on the
in-memory target list. The list-based check is only needed when the
properties are listed explicitly; when they are derived from the
table's attributes, names are already unique.

For labels, a single syscache probe on pg_propgraph_element_label
suffices for both the in-clause and cross-statement cases because
insert_element_record() already issues CommandCounterIncrement()
between successive label inserts.

Author: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/20260630173053.51.noahmisch%40microsoft.com

Branch
------
REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/22d0eebfef780b21c4e5a16966eff59fb7a9b9d6

Modified Files
--------------
src/backend/commands/propgraphcmds.c | 37 ++++++++++++++++++++--
.../regress/expected/create_property_graph.out | 14 ++++++++
src/test/regress/sql/create_property_graph.sql | 12 +++++++
3 files changed, 60 insertions(+), 3 deletions(-)