From 4b94ab372f45ff4c3d4d87db5bff318ef45e1f5e Mon Sep 17 00:00:00 2001
From: LeaFrock <petergeorge@126.com>
Date: Mon, 16 May 2022 20:53:22 +0800
Subject: [PATCH] Optimize `string.IndexOf(string, int)` translation when
 `startIndex` is a constant

---
 .../MySqlStringComparisonMethodTranslator.cs    |  9 ++++++---
 .../NorthwindFunctionsQueryMySqlTest.MySql.cs   | 17 ++++++++++++++++-
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/src/EFCore.MySql/Query/ExpressionTranslators/Internal/MySqlStringComparisonMethodTranslator.cs b/src/EFCore.MySql/Query/ExpressionTranslators/Internal/MySqlStringComparisonMethodTranslator.cs
index 0a53facf1..92de70a72 100644
--- a/src/EFCore.MySql/Query/ExpressionTranslators/Internal/MySqlStringComparisonMethodTranslator.cs
+++ b/src/EFCore.MySql/Query/ExpressionTranslators/Internal/MySqlStringComparisonMethodTranslator.cs
@@ -622,9 +622,12 @@ private SqlExpression CharLength(SqlExpression value)
 
         private SqlExpression Locate(SqlExpression sub, SqlExpression str, SqlExpression startIndex = null)
         {
-            var args = startIndex is null
-                ? new SqlExpression[] { sub, str }
-                : new SqlExpression[] { sub, str, _sqlExpressionFactory.Add(startIndex, _sqlExpressionFactory.Constant(1)) };
+            var args = startIndex switch
+            {
+                null => new SqlExpression[] { sub, str },
+                SqlConstantExpression { Value:int idx } => new SqlExpression[] { sub, str, _sqlExpressionFactory.Constant(idx + 1) },
+                _ => new SqlExpression[] { sub, str, _sqlExpressionFactory.Add(startIndex, _sqlExpressionFactory.Constant(1)) }
+            };
             return _sqlExpressionFactory.NullableFunction("LOCATE", args, typeof(int));
         }
 
diff --git a/test/EFCore.MySql.FunctionalTests/Query/NorthwindFunctionsQueryMySqlTest.MySql.cs b/test/EFCore.MySql.FunctionalTests/Query/NorthwindFunctionsQueryMySqlTest.MySql.cs
index fe7640637..06330029d 100644
--- a/test/EFCore.MySql.FunctionalTests/Query/NorthwindFunctionsQueryMySqlTest.MySql.cs
+++ b/test/EFCore.MySql.FunctionalTests/Query/NorthwindFunctionsQueryMySqlTest.MySql.cs
@@ -705,12 +705,27 @@ ELSE LOCATE(CONVERT(LCASE('nt') USING utf8mb4) COLLATE utf8mb4_bin, LCASE(`c`.`C
 END = 1");
         }
 
+        [ConditionalTheory]
+        [MemberData(nameof(IsAsyncData))]
+        public async Task StringIndexOf_with_constant_start_index(bool async)
+        {
+            await AssertQuery(
+                async,
+                ss => ss.Set<Customer>().Where(c => c.CustomerID.IndexOf("nt", 0, StringComparison.OrdinalIgnoreCase) == 1),
+                entryCount: 1);
+
+            AssertSql(
+                @"SELECT `c`.`CustomerID`, `c`.`Address`, `c`.`City`, `c`.`CompanyName`, `c`.`ContactName`, `c`.`ContactTitle`, `c`.`Country`, `c`.`Fax`, `c`.`Phone`, `c`.`PostalCode`, `c`.`Region`
+FROM `Customers` AS `c`
+WHERE (LOCATE(CONVERT(LCASE('nt') USING utf8mb4) COLLATE utf8mb4_bin, LCASE(`c`.`CustomerID`), 1) - 1) = 1");
+        }
+
         [ConditionalTheory]
         [InlineData(0, 1, false)]
         [InlineData(2, 0, false)]
         [InlineData(0, 1, true)]
         [InlineData(2, 0, true)]
-        public async Task StringIndexOf_with_start_index(int startIndex, int expected, bool async)
+        public async Task StringIndexOf_with_parameter_start_index(int startIndex, int expected, bool async)
         {
             await AssertQuery(
                 async,