Retrieves any constraints or keys (unique, pk, fk, check, index) across one or more columns.

def get_constraints(self, cursor, table_name):
        """
        Retrieves any constraints or keys (unique, pk, fk, check, index) across one or more columns.
        """
        constraints = {}
        # Loop over the key table, collecting things as constraints
        # This will get PKs, FKs, and uniques, but not CHECK
        cursor.execute("""
            SELECT
                kc.constraint_name,
                kc.column_name,
                c.constraint_type,
                array(SELECT table_name::text || '.' || column_name::text
                      FROM information_schema.constraint_column_usage
                      WHERE constraint_name = kc.constraint_name
                        AND constraint_schema = kc.constraint_schema)
            FROM information_schema.key_column_usage AS kc
            JOIN information_schema.table_constraints AS c ON
                kc.table_schema = c.table_schema AND
                kc.table_name = c.table_name AND
                kc.constraint_name = c.constraint_name
            WHERE
                kc.table_schema = %s AND
                kc.table_name = %s
            ORDER BY kc.ordinal_position ASC
        """, [self.connection.schema, table_name])
        for constraint, column, kind, used_cols in cursor.fetchall():
            # If we're the first column, make the record
            if constraint not in constraints:
                constraints[constraint] = {
                    "columns": [],
                    "primary_key": kind.lower() == "primary key",
                    "unique": kind.lower() in ["primary key", "unique"],
                    "foreign_key": tuple(used_cols[0].split(".", 1)) if kind.lower() == "foreign key" else None,
                    "check": False,
                    "index": False,
                }
            # Record the details
            constraints[constraint]['columns'].append(column)
        # Now get CHECK constraint columns
        cursor.execute("""
            SELECT kc.constraint_name, kc.column_name
            FROM information_schema.constraint_column_usage AS kc
            JOIN information_schema.table_constraints AS c ON
                kc.table_schema = c.table_schema AND
                kc.table_name = c.table_name AND
                kc.constraint_name = c.constraint_name
            WHERE
                c.constraint_type = 'CHECK' AND
                kc.table_schema = %s AND
                kc.table_name = %s
        """, [self.connection.schema, table_name])
        for constraint, column in cursor.fetchall():
            # If we're the first column, make the record
            if constraint not in constraints:
                constraints[constraint] = {
                    "columns": [],
                    "primary_key": False,
                    "unique": False,
                    "foreign_key": None,
                    "check": True,
                    "index": False,
                }
            # Record the details
            constraints[constraint]['columns'].append(column)
        # Now get indexes
        cursor.execute("""
            SELECT
                c2.relname,
                ARRAY(
                    SELECT (SELECT attname FROM pg_catalog.pg_attribute WHERE attnum = i AND attrelid = c.oid)
                    FROM unnest(idx.indkey) i
                ),
                idx.indisunique,
                idx.indisprimary
            FROM pg_catalog.pg_class c, pg_catalog.pg_class c2,
                pg_catalog.pg_index idx, pg_catalog.pg_namespace n
            WHERE c.oid = idx.indrelid
                AND idx.indexrelid = c2.oid
                AND c.relnamespace = n.oid
                AND n.nspname = %s
                AND c.relname = %s
        """, [self.connection.schema, table_name])
        for index, columns, unique, primary in cursor.fetchall():
            if index not in constraints:
                constraints[index] = {
                    "columns": list(columns),
                    "primary_key": primary,
                    "unique": unique,
                    "foreign_key": None,
                    "check": False,
                    "index": True,
                }
        return constraints