PostgreSQL query fails over PGNP provider for SQL Server - sql-server

Question

This query is failing in SQL Server over the PGNP provider:
SELECT 1 AS x
FROM pg_catalog.pg_attribute a
LEFT OUTER JOIN pg_catalog.pg_index i
ON (i.indrelid = a.attrelid
AND a.attnum = i.indkey[a.attnum-1])
This is the error message returned:
The OLE DB provider "PGNP" for linked server "XXX" indicates that either the object has no columns or the current user does not have permissions on that object.
If I run this query directly on the PostgreSQL server with pgAdmin, it returns results. Furthermore, if I remove the clause "a.attnum = i.indkey[a.attnum-1]", the query works over the PGNP provider. (I need this clause in my original query to identify which of a table's columns is the primary key.)
What is the issue, and how do I resolve it?
For the record, this is the complete query that I am executing, from which the query above was excerpted:
SELECT c.relname as table_name, a.attname as column_name
, t.typname as type_name
, i.indisunique AS is_unique_key
, i.indisprimary AS is_primary_key
, CASE t.typname
WHEN 'bpchar' THEN 'char('+CAST(atttypmod - 4 AS VARCHAR)+')'
WHEN 'numeric' THEN 'numeric('+CAST((atttypmod - 4)/65536 AS VARCHAR)
+','+CAST((atttypmod - 4)%65536 AS VARCHAR)
+')'
WHEN 'text' THEN 'varchar(max)'
WHEN 'varchar' THEN 'varchar('+CASE atttypmod WHEN -1 THEN 'max' ELSE CAST(atttypmod-4 AS VARCHAR) END+')'
ELSE t.typname
END as type_name_1
, CAST(CASE atttypmod WHEN -1 THEN NULL ELSE atttypmod - 4 END AS INT4) AS type_precision_scale
FROM pg_catalog.pg_attribute a
JOIN pg_catalog.pg_class c on ( a.attrelid = c.oid)
JOIN pg_catalog.pg_namespace n on (c.relnamespace = n.oid)
JOIN pg_catalog.pg_type t on (a.atttypid = t.oid)
LEFT OUTER JOIN pg_catalog.pg_index i
ON (c.oid = i.indrelid
AND i.indrelid = a.attrelid
AND a.attnum = i.indkey[a.attnum-1])
WHERE c.relkind = 'r' and a.attnum > 0
AND n.nspname = 'public'