Fix a bug where the result of a function that returns an array is not able to be accessed by array index.

src/EFCore.PG/Query/ExpressionTranslators/Internal/NpgsqlArrayMethodTranslator.cs

@@ -79,21 +79,28 @@ public NpgsqlArrayMethodTranslator(NpgsqlSqlExpressionFactory sqlExpressionFacto
         // During preprocessing, ArrayIndex and List[] get normalized to ElementAt; so we handle indexing into array/list here
         if (method.IsClosedFormOf(Enumerable_ElementAt))
         {
-            // Indexing over bytea is special, we have to use function rather than subscript
-            if (arguments[0].TypeMapping is NpgsqlByteArrayTypeMapping)
+            return arguments[0].TypeMapping switch
             {
-                return _sqlExpressionFactory.Function(
-                    "get_byte",
-                    [arguments[0], arguments[1]],
-                    nullable: true,
-                    argumentsPropagateNullability: TrueArrays[2],
-                    typeof(byte));
-            }
-
-            // Try translating indexing inside JSON column
-            // Note that Length over PG arrays (not within JSON) gets translated by QueryableMethodTranslatingEV, since arrays are primitive
-            // collections
-            return _jsonPocoTranslator.TranslateMemberAccess(arguments[0], arguments[1], method.ReturnType);
+                // Indexing over bytea is special, we have to use function rather than subscript
+                NpgsqlByteArrayTypeMapping
+                    => _sqlExpressionFactory.Function(
+                        "get_byte",
+                        [arguments[0], arguments[1]],
+                        nullable: true,
+                        argumentsPropagateNullability: TrueArrays[2],
+                        typeof(byte)),
+
+                NpgsqlArrayTypeMapping typeMapping
+                    => _sqlExpressionFactory.ArrayIndex(
+                        arguments[0],
+                        _sqlExpressionFactory.GenerateOneBasedIndexExpression(arguments[1]),
+                        nullable: true),
+
+                // Try translating indexing inside JSON column
+                // Note that Length over PG arrays (not within JSON) gets translated by QueryableMethodTranslatingEV, since arrays are primitive
+                // collections
+                _ => _jsonPocoTranslator.TranslateMemberAccess(arguments[0], arguments[1], method.ReturnType)
+            };
         }
 
         if (method.IsClosedFormOf(Enumerable_SequenceEqual)

src/EFCore.PG/Query/Internal/NpgsqlQuerySqlGenerator.cs

@@ -1485,6 +1485,9 @@ protected override bool RequiresParentheses(SqlExpression outerExpression, SqlEx
                 return true;
             }
 
+            // PG requires function calls to be wrapped in parentheses before indexing on the returned array:
+            // (string_to_array(c."ContactName", ' '))[1]
+            case SqlFunctionExpression when outerExpression is PgArrayIndexExpression:
             case PgUnknownBinaryExpression:
                 return true;
 

test/EFCore.PG.FunctionalTests/Query/NorthwindDbFunctionsQueryNpgsqlTest.cs

@@ -228,6 +228,28 @@ WHERE string_to_array(c."ContactName", ' ', 'Maria') = ARRAY[NULL,'Anders']::tex
 """);
     }
 
+    [Fact]
+    public void StringToArray_with_index()
+    {
+        using var context = CreateContext();
+        var count = context.Customers
+            .Select(c => EF.Functions.StringToArray(c.ContactName, " ")[0])
+            .Distinct()
+            .Count(c => c == "Maria");
+
+        Assert.Equal(1, count);
+
+        AssertSql(
+            """
+SELECT count(*)::int
+FROM (
+    SELECT DISTINCT (string_to_array(c."ContactName", ' '))[1] AS c
+    FROM "Customers" AS c
+    WHERE (string_to_array(c."ContactName", ' '))[1] = 'Maria'
+) AS c0
+""");
+    }
+
     [Fact]
     public void ToDate()
     {