Add CREATE COLLATION IF NOT EXISTS clause

The core of the functionality was already implemented when
pg_import_system_collations was added.  This just exposes it as an
option in the SQL command.

doc/src/sgml/ref/create_collation.sgml

@@ -18,12 +18,12 @@
 
  <refsynopsisdiv>
 <synopsis>
-CREATE COLLATION <replaceable>name</replaceable> (
+CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> (
     [ LOCALE = <replaceable>locale</replaceable>, ]
     [ LC_COLLATE = <replaceable>lc_collate</replaceable>, ]
     [ LC_CTYPE = <replaceable>lc_ctype</replaceable> ]
 )
-CREATE COLLATION <replaceable>name</replaceable> FROM <replaceable>existing_collation</replaceable>
+CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replaceable>existing_collation</replaceable>
 </synopsis>
  </refsynopsisdiv>
 
@@ -47,6 +47,17 @@ CREATE COLLATION <replaceable>name</replaceable> FROM <replaceable>existing_coll
   <title>Parameters</title>
 
    <variablelist>
+    <varlistentry>
+     <term><literal>IF NOT EXISTS</literal></term>
+     <listitem>
+      <para>
+       Do not throw an error if a collation with the same name already exists.
+       A notice is issued in this case. Note that there is no guarantee that
+       the existing collation is anything like the one that would have been created.
+      </para>
+     </listitem>
+    </varlistentry>
+
     <varlistentry>
      <term><replaceable>name</replaceable></term>
 

src/backend/commands/collationcmds.c

@@ -37,7 +37,7 @@
  * CREATE COLLATION
  */
 ObjectAddress
-DefineCollation(ParseState *pstate, List *names, List *parameters)
+DefineCollation(ParseState *pstate, List *names, List *parameters, bool if_not_exists)
 {
 	char	   *collName;
 	Oid			collNamespace;
@@ -137,7 +137,7 @@ DefineCollation(ParseState *pstate, List *names, List *parameters)
 							 GetDatabaseEncoding(),
 							 collcollate,
 							 collctype,
-							 false);
+							 if_not_exists);
 
 	if (!OidIsValid(newoid))
 		return InvalidObjectAddress;

src/backend/nodes/copyfuncs.c

@@ -3105,6 +3105,7 @@ _copyDefineStmt(const DefineStmt *from)
 	COPY_NODE_FIELD(defnames);
 	COPY_NODE_FIELD(args);
 	COPY_NODE_FIELD(definition);
+	COPY_SCALAR_FIELD(if_not_exists);
 
 	return newnode;
 }

src/backend/nodes/equalfuncs.c

@@ -1211,6 +1211,7 @@ _equalDefineStmt(const DefineStmt *a, const DefineStmt *b)
 	COMPARE_NODE_FIELD(defnames);
 	COMPARE_NODE_FIELD(args);
 	COMPARE_NODE_FIELD(definition);
+	COMPARE_SCALAR_FIELD(if_not_exists);
 
 	return true;
 }

src/backend/parser/gram.y

@@ -5610,6 +5610,16 @@ DefineStmt:
 					n->definition = $4;
 					$$ = (Node *)n;
 				}
+			| CREATE COLLATION IF_P NOT EXISTS any_name definition
+				{
+					DefineStmt *n = makeNode(DefineStmt);
+					n->kind = OBJECT_COLLATION;
+					n->args = NIL;
+					n->defnames = $6;
+					n->definition = $7;
+					n->if_not_exists = true;
+					$$ = (Node *)n;
+				}
 			| CREATE COLLATION any_name FROM any_name
 				{
 					DefineStmt *n = makeNode(DefineStmt);
@@ -5619,6 +5629,16 @@ DefineStmt:
 					n->definition = list_make1(makeDefElem("from", (Node *) $5, @5));
 					$$ = (Node *)n;
 				}
+			| CREATE COLLATION IF_P NOT EXISTS any_name FROM any_name
+				{
+					DefineStmt *n = makeNode(DefineStmt);
+					n->kind = OBJECT_COLLATION;
+					n->args = NIL;
+					n->defnames = $6;
+					n->definition = list_make1(makeDefElem("from", (Node *) $8, @8));
+					n->if_not_exists = true;
+					$$ = (Node *)n;
+				}
 		;
 
 definition: '(' def_list ')'						{ $$ = $2; }

src/backend/tcop/utility.c

@@ -1271,7 +1271,8 @@ ProcessUtilitySlow(ParseState *pstate,
 							Assert(stmt->args == NIL);
 							address = DefineCollation(pstate,
 													  stmt->defnames,
-													  stmt->definition);
+													  stmt->definition,
+													  stmt->if_not_exists);
 							break;
 						default:
 							elog(ERROR, "unrecognized define stmt type: %d",

src/include/commands/collationcmds.h

@@ -18,7 +18,7 @@
 #include "catalog/objectaddress.h"
 #include "nodes/parsenodes.h"
 
-extern ObjectAddress DefineCollation(ParseState *pstate, List *names, List *parameters);
+extern ObjectAddress DefineCollation(ParseState *pstate, List *names, List *parameters, bool if_not_exists);
 extern void IsThereCollationInNamespace(const char *collname, Oid nspOid);
 
 #endif   /* COLLATIONCMDS_H */

src/include/nodes/parsenodes.h

@@ -2380,6 +2380,7 @@ typedef struct DefineStmt
 	List	   *defnames;		/* qualified name (list of Value strings) */
 	List	   *args;			/* a list of TypeName (if needed) */
 	List	   *definition;		/* a list of DefElem */
+	bool		if_not_exists;	/* just do nothing if it already exists? */
 } DefineStmt;
 
 /* ----------------------

src/test/regress/expected/collate.linux.utf8.out

@@ -963,6 +963,10 @@ END
 $$;
 CREATE COLLATION test0 FROM "C"; -- fail, duplicate name
 ERROR:  collation "test0" for encoding "UTF8" already exists
+CREATE COLLATION IF NOT EXISTS test0 FROM "C"; -- ok, skipped
+NOTICE:  collation "test0" for encoding "UTF8" already exists, skipping
+CREATE COLLATION IF NOT EXISTS test0 (locale = 'foo'); -- ok, skipped
+NOTICE:  collation "test0" for encoding "UTF8" already exists, skipping
 do $$
 BEGIN
   EXECUTE 'CREATE COLLATION test1 (lc_collate = ' ||

src/test/regress/sql/collate.linux.utf8.sql

@@ -325,6 +325,8 @@ BEGIN
 END
 $$;
 CREATE COLLATION test0 FROM "C"; -- fail, duplicate name
+CREATE COLLATION IF NOT EXISTS test0 FROM "C"; -- ok, skipped
+CREATE COLLATION IF NOT EXISTS test0 (locale = 'foo'); -- ok, skipped
 do $$
 BEGIN
   EXECUTE 'CREATE COLLATION test1 (lc_collate = ' ||