Clean up partcollation handling for OID 0.

Consistent with what we do for indexes, we shouldn't try to record
dependencies on collation OID 0 or the default collation OID (which
is pinned).  Also, the fact that indcollation and partcollation can
contain zero OIDs when the data type is not collatable should be
documented.

Amit Langote, per a complaint from me.

Discussion: http://postgr.es/m/CA+Tgmoba5mtPgM3NKfG06vv8na5gGbVOj0h4zvivXQwLw8wXXQ@mail.gmail.com

doc/src/sgml/catalogs.sgml

@@ -3783,7 +3783,8 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</>:<replaceable>&lt;salt&gt;<
       <entry><literal><link linkend="catalog-pg-collation"><structname>pg_collation</structname></link>.oid</literal></entry>
       <entry>
        For each column in the index key, this contains the OID of the
-       collation to use for the index.
+       the collation to use for the index, or zero if the column is not
+       of a collatable data type.
       </entry>
      </row>
 
@@ -4770,7 +4771,8 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</>:<replaceable>&lt;salt&gt;<
       <entry><literal><link linkend="catalog-pg-opclass"><structname>pg_opclass</structname></link>.oid</literal></entry>
       <entry>
        For each column in the partition key, this contains the OID of the
-       the collation to use for partitioning.
+       the collation to use for partitioning, or zero if the column is not
+       of a collatable data type.
       </entry>
      </row>
 

src/backend/catalog/heap.c

@@ -3160,9 +3160,14 @@ StorePartitionKey(Relation rel,
 
 		recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL);
 
+		/* The default collation is pinned, so don't bother recording it */
+		if (OidIsValid(partcollation[i]) &&
+			partcollation[i] != DEFAULT_COLLATION_OID)
+		{
 			referenced.classId = CollationRelationId;
 			referenced.objectId = partcollation[i];
 			referenced.objectSubId = 0;
+		}
 
 		recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL);
 	}