Bug: trailing comma syntax error in postgres_fdw fetch_attstats()

[SATYANARAYANA NARLAPURAM]

Hi Hackers,

When importing statistics from a remote server running PostgreSQL < 17,
fetch_attstats() emits three NULL placeholders for the range statistics
columns that don't exist on older servers.  The string literal included a
trailing
comma after the last NULL, producing invalid SQL:

    SELECT ... NULL, NULL, NULL, FROM pg_catalog.pg_stats ...

This fails ANALYZE or IMPORT STATISTICS on a foreign table backed
by a pre-v17 remote server with a syntax error.

Thanks,
Satya

v1-0001-postgres_fdw-fix-trailing-comma.patch
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