pg_dump et al: Add --if-exists option

This option makes pg_dump, pg_dumpall and pg_restore inject an IF EXISTS
clause to each DROP command they emit.  (In pg_dumpall, the clause is
not added to individual objects drops, but rather to the CREATE DATABASE
commands, as well as CREATE ROLE and CREATE TABLESPACE.)

This allows for a better user dump experience when using --clean in case
some objects do not already exist.  Per bug #7873 by Dave Rolsky.

Author: Pavel Stěhule
Reviewed-by: Jeevan Chalke, Álvaro Herrera, Josh Kupershmidt

doc/src/sgml/ref/pg_dump.sgml

@@ -145,7 +145,8 @@ PostgreSQL documentation
        <para>
         Output commands to clean (drop)
         database objects prior to outputting the commands for creating them.
-        (Restore might generate some harmless error messages, if any objects
+        (Unless <option>--if-exists</> is also specified,
+        restore might generate some harmless error messages, if any objects
         were not present in the destination database.)
        </para>
 
@@ -649,6 +650,17 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--if-exists</option></term>
+      <listitem>
+       <para>
+        Use conditional commands (i.e. add an <literal>IF EXISTS</literal>
+        clause) when cleaning database objects.  This option is not valid
+        unless <option>--clean</> is also specified.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>--disable-dollar-quoting</></term>
       <listitem>

doc/src/sgml/ref/pg_dumpall.sgml

@@ -300,6 +300,17 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--if-exists</option></term>
+      <listitem>
+       <para>
+        Use conditional commands (i.e. add an <literal>IF EXISTS</literal>
+        clause) to clean databases and other objects.  This option is not valid
+        unless <option>--clean</> is also specified.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>--inserts</option></term>
       <listitem>

doc/src/sgml/ref/pg_restore.sgml

@@ -109,7 +109,8 @@
       <listitem>
        <para>
         Clean (drop) database objects before recreating them.
-        (This might generate some harmless error messages, if any objects
+        (Unless <option>--if-exists</> is used,
+        this might generate some harmless error messages, if any objects
         were not present in the destination database.)
        </para>
       </listitem>
@@ -489,6 +490,17 @@
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--if-exists</option></term>
+      <listitem>
+       <para>
+        Use conditional commands (i.e. add an <literal>IF EXISTS</literal>
+        clause) when cleaning database objects.  This option is not valid
+        unless <option>--clean</> is also specified.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>--no-data-for-failed-tables</option></term>
       <listitem>

src/bin/pg_dump/pg_backup.h

@@ -113,6 +113,7 @@ typedef struct _restoreOptions
 	char	   *superuser;		/* Username to use as superuser */
 	char	   *use_role;		/* Issue SET ROLE to this */
 	int			dropSchema;
+	int			if_exists;
 	const char *filename;
 	int			dataOnly;
 	int			schemaOnly;

src/bin/pg_dump/pg_backup_archiver.c

@@ -413,8 +413,77 @@ RestoreArchive(Archive *AHX)
 				/* Select owner and schema as necessary */
 				_becomeOwner(AH, te);
 				_selectOutputSchema(AH, te->namespace);
-				/* Drop it */
+
-				ahprintf(AH, "%s", te->dropStmt);
+				/*
+				 * Now emit the DROP command, if the object has one.  Note we
+				 * don't necessarily emit it verbatim; at this point we add an
+				 * appropriate IF EXISTS clause, if the user requested it.
+				 */
+				if (*te->dropStmt != '\0')
+				{
+					if (!ropt->if_exists)
+					{
+						/* No --if-exists?  Then just use the original */
+						ahprintf(AH, "%s", te->dropStmt);
+					}
+					else
+					{
+						char		buffer[40];
+						char	   *mark;
+						char	   *dropStmt = pg_strdup(te->dropStmt);
+						char	   *dropStmtPtr = dropStmt;
+						PQExpBuffer	ftStmt = createPQExpBuffer();
+
+						/*
+						 * Need to inject IF EXISTS clause after ALTER TABLE
+						 * part in ALTER TABLE .. DROP statement
+						 */
+						if (strncmp(dropStmt, "ALTER TABLE", 11) == 0)
+						{
+							appendPQExpBuffer(ftStmt,
+											  "ALTER TABLE IF EXISTS");
+							dropStmt = dropStmt + 11;
+						}
+
+						/*
+						 * ALTER TABLE..ALTER COLUMN..DROP DEFAULT does not
+						 * support the IF EXISTS clause, and therefore we
+						 * simply emit the original command for such objects.
+						 * For other objects, we need to extract the first part
+						 * of the DROP which includes the object type.  Most of
+						 * the time this matches te->desc, so search for that;
+						 * however for the different kinds of CONSTRAINTs, we
+						 * know to search for hardcoded "DROP CONSTRAINT"
+						 * instead.
+						 */
+						if (strcmp(te->desc, "DEFAULT") == 0)
+							appendPQExpBuffer(ftStmt, "%s", dropStmt);
+						else
+						{
+							if (strcmp(te->desc, "CONSTRAINT") == 0 ||
+								strcmp(te->desc, "CHECK CONSTRAINT") == 0 ||
+								strcmp(te->desc, "FK CONSTRAINT") == 0)
+								strcpy(buffer, "DROP CONSTRAINT");
+							else
+								snprintf(buffer, sizeof(buffer), "DROP %s",
+										 te->desc);
+
+							mark = strstr(dropStmt, buffer);
+							Assert(mark != NULL);
+
+							*mark = '\0';
+							appendPQExpBuffer(ftStmt, "%s%s IF EXISTS%s",
+											  dropStmt, buffer,
+											  mark + strlen(buffer));
+						}
+
+						ahprintf(AH, "%s", ftStmt->data);
+
+						destroyPQExpBuffer(ftStmt);
+
+						pg_free(dropStmtPtr);
+					}
+				}
 			}
 		}
 

src/bin/pg_dump/pg_dump.c

@@ -132,6 +132,7 @@ static int	binary_upgrade = 0;
 static int	disable_dollar_quoting = 0;
 static int	dump_inserts = 0;
 static int	column_inserts = 0;
+static int	if_exists = 0;
 static int	no_security_labels = 0;
 static int	no_synchronized_snapshots = 0;
 static int	no_unlogged_table_data = 0;
@@ -345,6 +346,7 @@ main(int argc, char **argv)
 		{"disable-dollar-quoting", no_argument, &disable_dollar_quoting, 1},
 		{"disable-triggers", no_argument, &disable_triggers, 1},
 		{"exclude-table-data", required_argument, NULL, 4},
+		{"if-exists", no_argument, &if_exists, 1},
 		{"inserts", no_argument, &dump_inserts, 1},
 		{"lock-wait-timeout", required_argument, NULL, 2},
 		{"no-tablespaces", no_argument, &outputNoTablespaces, 1},
@@ -573,6 +575,9 @@ main(int argc, char **argv)
 		exit_nicely(1);
 	}
 
+	if (if_exists && !outputClean)
+		exit_horribly(NULL, "option --if-exists requires -c/--clean option\n");
+
 	/* Identify archive format to emit */
 	archiveFormat = parseArchiveFormat(format, &archiveMode);
 
@@ -805,6 +810,7 @@ main(int argc, char **argv)
 	ropt->dropSchema = outputClean;
 	ropt->dataOnly = dataOnly;
 	ropt->schemaOnly = schemaOnly;
+	ropt->if_exists = if_exists;
 	ropt->dumpSections = dumpSections;
 	ropt->aclsSkip = aclsSkip;
 	ropt->superuser = outputSuperuser;
@@ -886,6 +892,7 @@ help(const char *progname)
 	printf(_("  --disable-dollar-quoting     disable dollar quoting, use SQL standard quoting\n"));
 	printf(_("  --disable-triggers           disable triggers during data-only restore\n"));
 	printf(_("  --exclude-table-data=TABLE   do NOT dump data for the named table(s)\n"));
+	printf(_("  --if-exists                  use IF EXISTS when dropping objects\n"));
 	printf(_("  --inserts                    dump data as INSERT commands, rather than COPY\n"));
 	printf(_("  --no-security-labels         do not dump security label assignments\n"));
 	printf(_("  --no-synchronized-snapshots  do not use synchronized snapshots in parallel jobs\n"));

src/bin/pg_dump/pg_dumpall.c

@@ -73,6 +73,7 @@ static int	binary_upgrade = 0;
 static int	column_inserts = 0;
 static int	disable_dollar_quoting = 0;
 static int	disable_triggers = 0;
+static int	if_exists = 0;
 static int	inserts = 0;
 static int	no_tablespaces = 0;
 static int	use_setsessauth = 0;
@@ -119,6 +120,7 @@ main(int argc, char *argv[])
 		{"column-inserts", no_argument, &column_inserts, 1},
 		{"disable-dollar-quoting", no_argument, &disable_dollar_quoting, 1},
 		{"disable-triggers", no_argument, &disable_triggers, 1},
+		{"if-exists", no_argument, &if_exists, 1},
 		{"inserts", no_argument, &inserts, 1},
 		{"lock-wait-timeout", required_argument, NULL, 2},
 		{"no-tablespaces", no_argument, &no_tablespaces, 1},
@@ -334,6 +336,13 @@ main(int argc, char *argv[])
 		exit_nicely(1);
 	}
 
+	if (if_exists && !output_clean)
+	{
+		fprintf(stderr, _("%s: option --if-exists requires -c/--clean option\n"),
+				progname);
+		exit_nicely(1);
+	}
+
 	if (roles_only && tablespaces_only)
 	{
 		fprintf(stderr, _("%s: options -r/--roles-only and -t/--tablespaces-only cannot be used together\n"),
@@ -564,6 +573,7 @@ help(void)
 	printf(_("  --column-inserts             dump data as INSERT commands with column names\n"));
 	printf(_("  --disable-dollar-quoting     disable dollar quoting, use SQL standard quoting\n"));
 	printf(_("  --disable-triggers           disable triggers during data-only restore\n"));
+	printf(_("  --if-exists                  use IF EXISTS when dropping objects\n"));
 	printf(_("  --inserts                    dump data as INSERT commands, rather than COPY\n"));
 	printf(_("  --no-security-labels         do not dump security label assignments\n"));
 	printf(_("  --no-tablespaces             do not dump tablespace assignments\n"));
@@ -624,7 +634,9 @@ dropRoles(PGconn *conn)
 
 		rolename = PQgetvalue(res, i, i_rolname);
 
-		fprintf(OPF, "DROP ROLE %s;\n", fmtId(rolename));
+		fprintf(OPF, "DROP ROLE %s%s;\n",
+				if_exists ? "IF EXISTS " : "",
+				fmtId(rolename));
 	}
 
 	PQclear(res);
@@ -994,7 +1006,9 @@ dropTablespaces(PGconn *conn)
 	{
 		char	   *spcname = PQgetvalue(res, i, 0);
 
-		fprintf(OPF, "DROP TABLESPACE %s;\n", fmtId(spcname));
+		fprintf(OPF, "DROP TABLESPACE %s%s;\n",
+				if_exists ? "IF EXISTS " : "",
+				fmtId(spcname));
 	}
 
 	PQclear(res);
@@ -1148,7 +1162,9 @@ dropDBs(PGconn *conn)
 		if (strcmp(dbname, "template1") != 0 &&
 			strcmp(dbname, "postgres") != 0)
 		{
-			fprintf(OPF, "DROP DATABASE %s;\n", fmtId(dbname));
+			fprintf(OPF, "DROP DATABASE %s%s;\n",
+					if_exists ? "IF EXISTS " : "",
+					fmtId(dbname));
 		}
 	}
 

src/bin/pg_dump/pg_restore.c

@@ -70,6 +70,7 @@ main(int argc, char **argv)
 	Archive    *AH;
 	char	   *inputFileSpec;
 	static int	disable_triggers = 0;
+	static int	if_exists = 0;
 	static int	no_data_for_failed_tables = 0;
 	static int	outputNoTablespaces = 0;
 	static int	use_setsessauth = 0;
@@ -110,6 +111,7 @@ main(int argc, char **argv)
 		 * the following options don't have an equivalent short option letter
 		 */
 		{"disable-triggers", no_argument, &disable_triggers, 1},
+		{"if-exists", no_argument, &if_exists, 1},
 		{"no-data-for-failed-tables", no_argument, &no_data_for_failed_tables, 1},
 		{"no-tablespaces", no_argument, &outputNoTablespaces, 1},
 		{"role", required_argument, NULL, 2},
@@ -336,6 +338,14 @@ main(int argc, char **argv)
 	opts->use_setsessauth = use_setsessauth;
 	opts->no_security_labels = no_security_labels;
 
+	if (if_exists && !opts->dropSchema)
+	{
+		fprintf(stderr, _("%s: option --if-exists requires -c/--clean option\n"),
+				progname);
+		exit_nicely(1);
+	}
+	opts->if_exists = if_exists;
+
 	if (opts->formatName)
 	{
 		switch (opts->formatName[0])
@@ -450,6 +460,7 @@ usage(const char *progname)
 	printf(_("  -x, --no-privileges          skip restoration of access privileges (grant/revoke)\n"));
 	printf(_("  -1, --single-transaction     restore as a single transaction\n"));
 	printf(_("  --disable-triggers           disable triggers during data-only restore\n"));
+	printf(_("  --if-exists                  use IF EXISTS when dropping objects\n"));
 	printf(_("  --no-data-for-failed-tables  do not restore data of tables that could not be\n"
 			 "                               created\n"));
 	printf(_("  --no-security-labels         do not restore security labels\n"));