From 7971893868e6eedc7229d28442f07890f251c42b Mon Sep 17 00:00:00 2001
From: Japin Li <japinli@hotmail.com>
Date: Sun, 27 Feb 2022 22:02:59 +0800
Subject: [PATCH v1] Add CREATE DATABASE IF NOT EXISTS syntax

---
 doc/src/sgml/ref/create_database.sgml |  2 +-
 src/backend/commands/dbcommands.c     | 19 +++++++++++++++----
 src/backend/nodes/copyfuncs.c         |  1 +
 src/backend/nodes/equalfuncs.c        |  1 +
 src/backend/parser/gram.y             |  9 +++++++++
 src/include/nodes/parsenodes.h        |  1 +
 6 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/doc/src/sgml/ref/create_database.sgml b/doc/src/sgml/ref/create_database.sgml
index f70d0c75b4..74af9c586e 100644
--- a/doc/src/sgml/ref/create_database.sgml
+++ b/doc/src/sgml/ref/create_database.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-CREATE DATABASE <replaceable class="parameter">name</replaceable>
+CREATE DATABASE <replaceable class="parameter">name</replaceable> [ IF NOT EXISTS ]
     [ [ WITH ] [ OWNER [=] <replaceable class="parameter">user_name</replaceable> ]
            [ TEMPLATE [=] <replaceable class="parameter">template</replaceable> ]
            [ ENCODING [=] <replaceable class="parameter">encoding</replaceable> ]
diff --git a/src/backend/commands/dbcommands.c b/src/backend/commands/dbcommands.c
index c37e3c9a9a..f3e5e930f9 100644
--- a/src/backend/commands/dbcommands.c
+++ b/src/backend/commands/dbcommands.c
@@ -557,10 +557,21 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	 * message than "unique index violation".  There's a race condition but
 	 * we're willing to accept the less friendly message in that case.
 	 */
-	if (OidIsValid(get_database_oid(dbname, true)))
-		ereport(ERROR,
-				(errcode(ERRCODE_DUPLICATE_DATABASE),
-				 errmsg("database \"%s\" already exists", dbname)));
+	{
+		Oid	check_dboid = get_database_oid(dbname, true);
+		if (OidIsValid(check_dboid))
+		{
+			if (!stmt->if_not_exists)
+				ereport(ERROR,
+					(errcode(ERRCODE_DUPLICATE_DATABASE),
+					 errmsg("database \"%s\" already exists", dbname)));
+
+			ereport(NOTICE,
+					(errcode(ERRCODE_DUPLICATE_DATABASE),
+					 errmsg("database \"%s\" already exists, skipping", dbname)));
+			return check_dboid;
+		}
+	}
 
 	/*
 	 * The source DB can't have any active backends, except this one
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index d4f8455a2b..83ead22931 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -4045,6 +4045,7 @@ _copyCreatedbStmt(const CreatedbStmt *from)
 
 	COPY_STRING_FIELD(dbname);
 	COPY_NODE_FIELD(options);
+	COPY_SCALAR_FIELD(if_not_exists);
 
 	return newnode;
 }
diff --git a/src/backend/nodes/equalfuncs.c b/src/backend/nodes/equalfuncs.c
index f1002afe7a..f9a89fa4a8 100644
--- a/src/backend/nodes/equalfuncs.c
+++ b/src/backend/nodes/equalfuncs.c
@@ -1722,6 +1722,7 @@ _equalCreatedbStmt(const CreatedbStmt *a, const CreatedbStmt *b)
 {
 	COMPARE_STRING_FIELD(dbname);
 	COMPARE_NODE_FIELD(options);
+	COMPARE_SCALAR_FIELD(if_not_exists);
 
 	return true;
 }
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index a03b33b53b..72e4f642d9 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -10395,6 +10395,15 @@ CreatedbStmt:
 					CreatedbStmt *n = makeNode(CreatedbStmt);
 					n->dbname = $3;
 					n->options = $5;
+					n->if_not_exists = false;
+					$$ = (Node *)n;
+				}
+			| CREATE DATABASE IF_P NOT EXISTS name opt_with createdb_opt_list
+				{
+					CreatedbStmt *n = makeNode(CreatedbStmt);
+					n->dbname = $6;
+					n->options = $8;
+					n->if_not_exists = true;
 					$$ = (Node *)n;
 				}
 		;
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 1617702d9d..71c828ac4d 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -3295,6 +3295,7 @@ typedef struct CreatedbStmt
 	NodeTag		type;
 	char	   *dbname;			/* name of database to create */
 	List	   *options;		/* List of DefElem nodes */
+	bool        if_not_exists;  /* just do nothing if it already exists? */
 } CreatedbStmt;
 
 /* ----------------------
-- 
2.25.1