fix: DROP function before CREATE when return type or param names change (#326)

PostgreSQL does not allow CREATE OR REPLACE FUNCTION to change the return
type or parameter names of an existing function. When these change (but input
parameter types stay the same, keeping the same function signature key),
pgschema now generates DROP FUNCTION followed by CREATE FUNCTION instead of
just CREATE OR REPLACE FUNCTION.

Also extracts generateDropFunctionSQL helper to share DROP statement generation
between the recreate path and the existing drop path.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: tianzhou

internal/diff/function.go

@@ -100,6 +100,31 @@ func generateModifyFunctionsSQL(diffs []*functionDiff, targetSchema string, coll
 			if oldFunc.Comment != newFunc.Comment {
 				generateFunctionComment(newFunc, targetSchema, DiffTypeFunction, DiffOperationAlter, collector)
 			}
+		} else if functionRequiresRecreate(oldFunc, newFunc) {
+			// Return type, OUT parameters, or parameter names changed - must DROP then CREATE
+			// PostgreSQL does not allow CREATE OR REPLACE to change these.
+			// See https://github.com/pgplex/pgschema/issues/326
+			dropSQL := generateDropFunctionSQL(oldFunc, targetSchema)
+			createSQL := generateFunctionSQL(newFunc, targetSchema)
+
+			alterContext := &diffContext{
+				Type:                DiffTypeFunction,
+				Operation:           DiffOperationAlter,
+				Path:                fmt.Sprintf("%s.%s", newFunc.Schema, newFunc.Name),
+				Source:              diff,
+				CanRunInTransaction: true,
+			}
+
+			statements := []SQLStatement{
+				{SQL: dropSQL, CanRunInTransaction: true},
+				{SQL: createSQL, CanRunInTransaction: true},
+			}
+			collector.collectStatements(alterContext, statements)
+
+			// Check if comment also changed alongside body changes
+			if oldFunc.Comment != newFunc.Comment {
+				generateFunctionComment(newFunc, targetSchema, DiffTypeFunction, DiffOperationAlter, collector)
+			}
 		} else {
 			// Function body or other attributes changed - use CREATE OR REPLACE
 			sql := generateFunctionSQL(newFunc, targetSchema)
@@ -129,17 +154,7 @@ func generateDropFunctionsSQL(functions []*ir.Function, targetSchema string, col
 	sortedFunctions := reverseSlice(topologicallySortFunctions(functions))
 
 	for _, function := range sortedFunctions {
-		functionName := qualifyEntityName(function.Schema, function.Name, targetSchema)
-		var sql string
-
-		// Build argument list for DROP statement using GetArguments()
-		argsList := function.GetArguments()
-
-		if argsList != "" {
-			sql = fmt.Sprintf("DROP FUNCTION IF EXISTS %s(%s);", functionName, argsList)
-		} else {
-			sql = fmt.Sprintf("DROP FUNCTION IF EXISTS %s();", functionName)
-		}
+		sql := generateDropFunctionSQL(function, targetSchema)
 
 		// Create context for this statement
 		context := &diffContext{
@@ -154,6 +169,16 @@ func generateDropFunctionsSQL(functions []*ir.Function, targetSchema string, col
 	}
 }
 
+// generateDropFunctionSQL generates a DROP FUNCTION IF EXISTS statement
+func generateDropFunctionSQL(function *ir.Function, targetSchema string) string {
+	functionName := qualifyEntityName(function.Schema, function.Name, targetSchema)
+	argsList := function.GetArguments()
+	if argsList != "" {
+		return fmt.Sprintf("DROP FUNCTION IF EXISTS %s(%s);", functionName, argsList)
+	}
+	return fmt.Sprintf("DROP FUNCTION IF EXISTS %s();", functionName)
+}
+
 // generateFunctionSQL generates CREATE OR REPLACE FUNCTION SQL for a function
 func generateFunctionSQL(function *ir.Function, targetSchema string) string {
 	var stmt strings.Builder
@@ -460,6 +485,27 @@ func functionsEqualExceptComment(old, new *ir.Function) bool {
 	return parametersEqual(oldInputParams, newInputParams)
 }
 
+// functionRequiresRecreate checks if a function modification requires DROP+CREATE
+// instead of CREATE OR REPLACE. PostgreSQL does not allow CREATE OR REPLACE to change
+// the return type or parameter names of an existing function.
+func functionRequiresRecreate(old, new *ir.Function) bool {
+	if old.ReturnType != new.ReturnType {
+		return true
+	}
+	// Check if parameter names changed (types are the same since same map key)
+	oldParams := filterNonTableParameters(old.Parameters)
+	newParams := filterNonTableParameters(new.Parameters)
+	if len(oldParams) != len(newParams) {
+		return true
+	}
+	for i := range oldParams {
+		if oldParams[i].Name != newParams[i].Name {
+			return true
+		}
+	}
+	return false
+}
+
 // filterNonTableParameters filters out TABLE mode parameters
 // TABLE parameters are output columns in RETURNS TABLE() and shouldn't be compared as input parameters
 func filterNonTableParameters(params []*ir.Parameter) []*ir.Parameter {

testdata/diff/create_function/issue_326_param_name_change/diff.sql

@@ -0,0 +1,10 @@
+DROP FUNCTION IF EXISTS somefunction(text);
+
+CREATE OR REPLACE FUNCTION somefunction(
+    new_name text
+)
+RETURNS text
+LANGUAGE sql
+VOLATILE
+AS $$ SELECT new_name;
+$$;

testdata/diff/create_function/issue_326_param_name_change/new.sql

@@ -0,0 +1,5 @@
+CREATE OR REPLACE FUNCTION somefunction(
+    new_name text
+) RETURNS text
+LANGUAGE sql
+AS $$ SELECT new_name; $$;

testdata/diff/create_function/issue_326_param_name_change/old.sql

@@ -0,0 +1,5 @@
+CREATE OR REPLACE FUNCTION somefunction(
+    old_name text
+) RETURNS text
+LANGUAGE sql
+AS $$ SELECT old_name; $$;

testdata/diff/create_function/issue_326_param_name_change/plan.json

@@ -0,0 +1,26 @@
+{
+  "version": "1.0.0",
+  "pgschema_version": "1.7.2",
+  "created_at": "1970-01-01T00:00:00Z",
+  "source_fingerprint": {
+    "hash": "d87f2cfffc1d1273ca588466e14557b2698607c55dc7c8a0e44317046e3c95a9"
+  },
+  "groups": [
+    {
+      "steps": [
+        {
+          "sql": "DROP FUNCTION IF EXISTS somefunction(text);",
+          "type": "function",
+          "operation": "alter",
+          "path": "public.somefunction"
+        },
+        {
+          "sql": "CREATE OR REPLACE FUNCTION somefunction(\n    new_name text\n)\nRETURNS text\nLANGUAGE sql\nVOLATILE\nAS $$ SELECT new_name;\n$$;",
+          "type": "function",
+          "operation": "alter",
+          "path": "public.somefunction"
+        }
+      ]
+    }
+  ]
+}

testdata/diff/create_function/issue_326_param_name_change/plan.sql

@@ -0,0 +1,10 @@
+DROP FUNCTION IF EXISTS somefunction(text);
+
+CREATE OR REPLACE FUNCTION somefunction(
+    new_name text
+)
+RETURNS text
+LANGUAGE sql
+VOLATILE
+AS $$ SELECT new_name;
+$$;

testdata/diff/create_function/issue_326_param_name_change/plan.txt

@@ -0,0 +1,21 @@
+Plan: 1 to modify.
+
+Summary by type:
+  functions: 1 to modify
+
+Functions:
+  ~ somefunction
+
+DDL to be executed:
+--------------------------------------------------
+
+DROP FUNCTION IF EXISTS somefunction(text);
+
+CREATE OR REPLACE FUNCTION somefunction(
+    new_name text
+)
+RETURNS text
+LANGUAGE sql
+VOLATILE
+AS $$ SELECT new_name;
+$$;

testdata/diff/create_function/issue_326_param_type_change/diff.sql

@@ -0,0 +1,10 @@
+DROP FUNCTION IF EXISTS somefunction(text);
+
+CREATE OR REPLACE FUNCTION somefunction(
+    param2 uuid
+)
+RETURNS uuid
+LANGUAGE sql
+VOLATILE
+AS $$ SELECT param2;
+$$;

testdata/diff/create_function/issue_326_param_type_change/new.sql

@@ -0,0 +1,5 @@
+CREATE OR REPLACE FUNCTION somefunction(
+    param2 uuid
+) RETURNS uuid
+LANGUAGE sql
+AS $$ SELECT param2; $$;

testdata/diff/create_function/issue_326_param_type_change/old.sql

@@ -0,0 +1,5 @@
+CREATE OR REPLACE FUNCTION somefunction(
+    param1 text
+) RETURNS text
+LANGUAGE sql
+AS $$ SELECT param1; $$;