Prevent dropping distribution keys for MODULO

Due to a coding issue in IsDistColumnForRelId() it was possible to drop
columns that were used as modulo distribution keys. A simple example
demonstrates the behavior:

    CREATE TABLE t (a INT, b INT, c INT) DISTRIBUTE BY MODULO (a);
    ALTER TABLE t DROP COLUMN a;

    test=# \d+ t
                              Table "public.t"
     Column |  Type   | Modifiers | Storage | Stats target | Description
    --------+---------+-----------+---------+--------------+-------------
     b      | integer |           | plain   |              |
     c      | integer |           | plain   |              |
    Distribute By: MODULO(........pg.dropped.1........)
    Location Nodes: ALL DATANODES

With this commit, the ALTER TABLE command fails as expected:

    ERROR:  Distribution column cannot be dropped

The commit simplifies the coding a bit, and removes several functions
that were not needed anymore (and unused outside locator.c).

4 files changed, 17 insertions, 74 deletions

diff --git a/src/backend/pgxc/locator/locator.c b/src/backend/pgxc/locator/locator.c
index 2ecffe45ac..2c8aa676ae 100644
--- a/src/backend/pgxc/locator/locator.c
+++ b/src/backend/pgxc/locator/locator.c
@@ -261,54 +261,28 @@ GetRelationHashColumn(RelationLocInfo * rel_loc_info)
 }
 
 /*
- * IsHashColumn - return whether or not column for relation is hashed.
- *
- */
-bool
-IsHashColumn(RelationLocInfo *rel_loc_info, char *part_col_name)
-{
-	bool		ret_value = false;
-
-	if (!rel_loc_info || !part_col_name)
-		ret_value = false;
-	else if (rel_loc_info->locatorType != LOCATOR_TYPE_HASH)
-		ret_value = false;
-	else
-		ret_value = !strcmp(part_col_name, rel_loc_info->partAttrName);
-
-	return ret_value;
-}
-
-
-/*
- * IsHashColumnForRelId - return whether or not column for relation is hashed.
- *
- */
-bool
-IsHashColumnForRelId(Oid relid, char *part_col_name)
-{
-	RelationLocInfo *rel_loc_info = GetRelationLocInfo(relid);
-
-	return IsHashColumn(rel_loc_info, part_col_name);
-}
-
-/*
  * IsDistColumnForRelId - return whether or not column for relation is used for hash or modulo distribution
  *
  */
 bool
 IsDistColumnForRelId(Oid relid, char *part_col_name)
 {
-	bool bRet;
 	RelationLocInfo *rel_loc_info;
 
-	rel_loc_info = GetRelationLocInfo(relid);
-	bRet = false;
+	/* if no column is specified, we're done */
+	if (!part_col_name)
+		return false;
+
+	/* if no locator, we're done too */
+	if (!(rel_loc_info = GetRelationLocInfo(relid)))
+		return false;
+
+	/* is the table distributed by column value */
+	if (!IsRelationDistributedByValue(rel_loc_info))
+		return false;
 
-	bRet = IsHashColumn(rel_loc_info, part_col_name);
-	if (bRet == false)
-		IsModuloColumn(rel_loc_info, part_col_name);
-	return bRet;
+	/* does the column name match the distribution column */
+	return !strcmp(part_col_name, rel_loc_info->partAttrName);
 }
 
 
@@ -348,37 +322,6 @@ GetRelationModuloColumn(RelationLocInfo * rel_loc_info)
 }
 
 /*
- * IsModuloColumn - return whether or not column for relation is used for modulo distribution.
- *
- */
-bool
-IsModuloColumn(RelationLocInfo *rel_loc_info, char *part_col_name)
-{
-	bool		ret_value = false;
-
-	if (!rel_loc_info || !part_col_name)
-		ret_value = false;
-	else if (rel_loc_info->locatorType != LOCATOR_TYPE_MODULO)
-		ret_value = false;
-	else
-		ret_value = !strcmp(part_col_name, rel_loc_info->partAttrName);
-
-	return ret_value;
-}
-
-
-/*
- * IsModuloColumnForRelId - return whether or not column for relation is used for modulo distribution.
- */
-bool
-IsModuloColumnForRelId(Oid relid, char *part_col_name)
-{
-	RelationLocInfo *rel_loc_info = GetRelationLocInfo(relid);
-
-	return IsModuloColumn(rel_loc_info, part_col_name);
-}
-
-/*
  * Update the round robin node for the relation
  *
  * PGXCTODO - may not want to bother with locking here, we could track
diff --git a/src/include/pgxc/locator.h b/src/include/pgxc/locator.h
index 5062cebe83..21c7e3fe1d 100644
--- a/src/include/pgxc/locator.h
+++ b/src/include/pgxc/locator.h
@@ -162,8 +162,6 @@ extern RelationLocInfo *CopyRelationLocInfo(RelationLocInfo *src_info);
 extern char GetRelationLocType(Oid relid);
 extern bool IsTableDistOnPrimary(RelationLocInfo *rel_loc_info);
 extern bool IsLocatorInfoEqual(RelationLocInfo *rel_loc_info1, RelationLocInfo *rel_loc_info2);
-extern bool IsHashColumn(RelationLocInfo *rel_loc_info, char *part_col_name);
-extern bool IsHashColumnForRelId(Oid relid, char *part_col_name);
 extern int	GetRoundRobinNode(Oid relid);
 extern ExecNodes *GetRelationNodes(RelationLocInfo *rel_loc_info,
 								   Datum valueForDistCol,
@@ -184,8 +182,6 @@ extern void FreeRelationLocInfo(RelationLocInfo *relationLocInfo);
 
 extern bool IsTypeModuloDistributable(Oid col_type);
 extern char *GetRelationModuloColumn(RelationLocInfo *rel_loc_info);
-extern bool IsModuloColumn(RelationLocInfo *rel_loc_info, char *part_col_name);
-extern bool IsModuloColumnForRelId(Oid relid, char *part_col_name);
 extern char *GetRelationDistColumn(RelationLocInfo *rel_loc_info);
 extern bool IsDistColumnForRelId(Oid relid, char *part_col_name);
 extern void FreeExecNodes(ExecNodes **exec_nodes);
diff --git a/src/test/regress/expected/xl_alter_table.out b/src/test/regress/expected/xl_alter_table.out
index 6d578b898a..edfe8b6d13 100755
--- a/src/test/regress/expected/xl_alter_table.out
+++ b/src/test/regress/expected/xl_alter_table.out
@@ -82,6 +82,8 @@ CREATE TABLE xl_at2m (
     product_no INT8,
     product_id INT2
 ) DISTRIBUTE BY MODULO (product_id); 
+ALTER TABLE xl_at2m DROP COLUMN product_id;--fail - distribution column cannot be dropped. 
+ERROR:  Distribution column cannot be dropped
 ALTER TABLE xl_at2m DISTRIBUTE BY HASH(product_id);
 ALTER TABLE xl_at2m DISTRIBUTE BY MODULO(product_id);
 ALTER TABLE xl_at2m DROP COLUMN product_no;
diff --git a/src/test/regress/sql/xl_alter_table.sql b/src/test/regress/sql/xl_alter_table.sql
index 1f1c112b2b..5e2477cc5e 100755
--- a/src/test/regress/sql/xl_alter_table.sql
+++ b/src/test/regress/sql/xl_alter_table.sql
@@ -137,6 +137,8 @@ CREATE TABLE xl_at2m (
     product_id INT2
 ) DISTRIBUTE BY MODULO (product_id); 
 
+ALTER TABLE xl_at2m DROP COLUMN product_id;--fail - distribution column cannot be dropped. 
+
 ALTER TABLE xl_at2m DISTRIBUTE BY HASH(product_id);
 
 ALTER TABLE xl_at2m DISTRIBUTE BY MODULO(product_id);