Maps additional array (IList) operators

| C# expression                    | SQL generated by Npgsql |
|----------------------------------|-------------------------|
| `Array.Length`                   | `array_length(1, 1)`
| `IList.Count`                    | `array_length(a, 1)`
| `Enumerable.Count(a)`            | `array_length(a, 1)`
| `Array[0]`                       | `a[1]`
| `IList[0]`                       | `a[1]`
| `Enumerable.ElementAt(a, 0)`     | `a[1]`
| `Enumerable.Append(a, b)`        | `a || b`
| `Enumerable.Concat(a, b)`        | `a || b`
| `Enumerable.Prepend(a, b)`       | `b || a`
| `Object.Equals(a, b)`            | `a = b`
| `Enumerable.SequenceEqual(a, b)` | `a = b`
| `a.ToString()`                   | `array_to_string(a, ',')`
| `Enumerable.IndexOf(a, 0)`       | `COALESCE(array_position(a, 1), -1)`
| `Enumerable.Contains(a, b)`      | `b = ANY (a)`

| C# expression              | SQL generated by Npgsql |
|----------------------------|-------------------------|
| `a.All(x => b.Contains(x)` | `b <@ a`
| `a.Any(x => b.Contains(x)` | `a && b`

- Relational operators could also be supported via extensions on `EF.Functions`.

- https://www.postgresql.org/docs/current/static/functions-array.html#ARRAY-OPERATORS-TABLE

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

@@ -40,7 +40,6 @@ public class NpgsqlCompositeMethodCallTranslator : RelationalCompositeMethodCall
         /// </summary>
         static readonly IMethodCallTranslator[] MethodCallTranslators =
         {
-            new NpgsqlArraySequenceEqualTranslator(),
             new NpgsqlConvertTranslator(),
             new NpgsqlDateAddTranslator(),
             new NpgsqlStringSubstringTranslator(),

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

@@ -25,10 +25,13 @@
 
 using System.Collections;
 using System.Collections.Generic;
+using System.Linq;
 using System.Linq.Expressions;
 using System.Reflection;
 using JetBrains.Annotations;
+using Microsoft.EntityFrameworkCore.Query.Expressions;
 using Microsoft.EntityFrameworkCore.Query.ExpressionTranslators;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Query.Expressions.Internal;
 
 namespace Npgsql.EntityFrameworkCore.PostgreSQL.Query.ExpressionTranslators.Internal
 {
@@ -44,45 +47,55 @@ public class NpgsqlListTranslator : IMethodCallTranslator
         [CanBeNull]
         public Expression Translate(MethodCallExpression expression)
         {
-            if (!typeof(IList).IsAssignableFrom(expression.Method.DeclaringType))
+            if (expression.Object != null && !typeof(IList).IsAssignableFrom(expression.Object.Type))
                 return null;
+            if (expression.Object == null && expression.Arguments.Count > 0 && !typeof(IList).IsAssignableFrom(expression.Arguments[0].Type))
+                return null;
+
+            // TODO: use #430 to map @> to source.All(x => other.Contains(x));
+            // TODO: use #430 to map && to soucre.Any(x => other.Contains(x));
 
             switch (expression.Method.Name)
             {
+            // TODO: Currently handled in NpgsqlSqlTranslatingExpressionVisitor.
+            case nameof(Enumerable.Append):
+            case nameof(Enumerable.Concat):
+                return new CustomBinaryExpression(expression.Arguments[0], expression.Arguments[1], "||", expression.Arguments[0].Type);
+
+            // TODO: Currently handled in NpgsqlSqlTranslatingExpressionVisitor.
+            case nameof(Enumerable.Count):
+                return new SqlFunctionExpression("array_length", typeof(int), new[] { expression.Arguments[0], Expression.Constant(1) });
+
             case "get_Item" when expression.Object is Expression instance:
                 return Expression.MakeIndex(instance, instance.Type.GetRuntimeProperty("Item"), expression.Arguments);
-//            case nameof(NpgsqlListExtensions.Contains):
-//                return new CustomBinaryExpression(expression.Arguments[0], expression.Arguments[1], "@>", typeof(bool));
-//
-//            case nameof(NpgsqlListExtensions.ContainedBy):
-//                return new CustomBinaryExpression(expression.Arguments[0], expression.Arguments[1], "<@", typeof(bool));
-//
-//            case nameof(NpgsqlListExtensions.Overlaps):
-//                return new CustomBinaryExpression(expression.Arguments[0], expression.Arguments[1], "&&", typeof(bool));
-//
-//            case nameof(NpgsqlListExtensions.IsStrictlyLeftOf):
-//                return new CustomBinaryExpression(expression.Arguments[0], expression.Arguments[1], "<<", typeof(bool));
-//
-//            case nameof(NpgsqlListExtensions.IsStrictlyRightOf):
-//                return new CustomBinaryExpression(expression.Arguments[0], expression.Arguments[1], ">>", typeof(bool));
-//
-//            case nameof(NpgsqlListExtensions.DoesNotExtendRightOf):
-//                return new CustomBinaryExpression(expression.Arguments[0], expression.Arguments[1], "&<", typeof(bool));
-//
-//            case nameof(NpgsqlListExtensions.DoesNotExtendLeftOf):
-//                return new CustomBinaryExpression(expression.Arguments[0], expression.Arguments[1], "&>", typeof(bool));
-//
-//            case nameof(NpgsqlListExtensions.IsAdjacentTo):
-//                return new CustomBinaryExpression(expression.Arguments[0], expression.Arguments[1], "-|-", typeof(bool));
-//
-//            case nameof(NpgsqlListExtensions.Union):
-//                return new CustomBinaryExpression(expression.Arguments[0], expression.Arguments[1], "+", expression.Arguments[0].Type);
-//
-//            case nameof(NpgsqlListExtensions.Intersect):
-//                return new CustomBinaryExpression(expression.Arguments[0], expression.Arguments[1], "*", expression.Arguments[0].Type);
-//
-//            case nameof(NpgsqlListExtensions.Except):
-//                return new CustomBinaryExpression(expression.Arguments[0], expression.Arguments[1], "-", expression.Arguments[0].Type);
+
+            case nameof(Enumerable.ElementAt):
+                return Expression.MakeIndex(expression.Arguments[0], expression.Arguments[0].Type.GetRuntimeProperty("Item"), new[] { expression.Arguments[1] });
+
+            case nameof(Enumerable.Prepend):
+                return new CustomBinaryExpression(expression.Arguments[1], expression.Arguments[0], "||", expression.Arguments[0].Type);
+
+            case nameof(Enumerable.SequenceEqual):
+                return Expression.MakeBinary(ExpressionType.Equal, expression.Arguments[0], expression.Arguments[1]);
+
+            case nameof(ToString):
+                return new SqlFunctionExpression("array_to_string", typeof(string), new[] { expression.Object, Expression.Constant(",") });
+
+            case nameof(IList.IndexOf):
+                return
+                    new SqlFunctionExpression(
+                        "COALESCE",
+                        typeof(int),
+                        new Expression[]
+                        {
+                            new SqlFunctionExpression(
+                                "array_position",
+                                typeof(int),
+                                expression.Object is null
+                                    ? (IEnumerable<Expression>)expression.Arguments
+                                    : new[] { expression.Object, expression.Arguments[0] }),
+                            Expression.Constant(-1)
+                        });
 
             default:
                 return null;

src/EFCore.PG/Query/ExpressionVisitors/NpgsqlSqlTranslatingExpressionVisitor.cs

@@ -76,6 +76,11 @@ protected override Expression VisitSubQuery(SubQueryExpression expression)
             var lastPropertyType = properties[properties.Count - 1].ClrType;
             if (typeof(IList).IsAssignableFrom(lastPropertyType) && subQueryModel.ResultOperators.Count > 0)
             {
+                if (subQueryModel.ResultOperators.First() is ConcatResultOperator concatResultOperator &&
+                    Visit(fromExpression) is Expression first &&
+                    Visit(concatResultOperator.Source2) is Expression second)
+                    return new CustomBinaryExpression(first, second, "||", first.Type);
+
                 // Translate someArray.Length
                 if (subQueryModel.ResultOperators.First() is CountResultOperator)
                     return new SqlFunctionExpression("array_length", typeof(int), new[] { Visit(fromExpression), Expression.Constant(1) });