expression: pushdown substring/char_length to tikv

cmd/explaintest/r/tpch.result

@@ -1294,14 +1294,13 @@ cntrycode
 order by
 cntrycode;
 id	estRows	task	access object	operator info
-Sort	1.00	root		Column#27
-└─Projection	1.00	root		Column#27, Column#28, Column#29
-  └─HashAgg	1.00	root		group by:Column#33, funcs:count(1)->Column#28, funcs:sum(Column#31)->Column#29, funcs:firstrow(Column#32)->Column#27
-    └─Projection	0.00	root		tpch.customer.c_acctbal, substring(tpch.customer.c_phone, 1, 2)->Column#32, substring(tpch.customer.c_phone, 1, 2)->Column#33
+Sort	1.00	root		Column#31
+└─Projection	1.00	root		Column#31, Column#32, Column#33
+  └─HashAgg	1.00	root		group by:Column#37, funcs:count(1)->Column#32, funcs:sum(Column#35)->Column#33, funcs:firstrow(Column#36)->Column#31
+    └─Projection	0.00	root		tpch.customer.c_acctbal, substring(tpch.customer.c_phone, 1, 2)->Column#36, substring(tpch.customer.c_phone, 1, 2)->Column#37
       └─HashJoin	0.00	root		anti semi join, equal:[eq(tpch.customer.c_custkey, tpch.orders.o_custkey)]
         ├─TableReader(Build)	75000000.00	root		data:TableFullScan
         │ └─TableFullScan	75000000.00	cop[tikv]	table:orders	keep order:false
-        └─Selection(Probe)	0.00	root		in(substring(tpch.customer.c_phone, 1, 2), "20", "40", "22", "30", "39", "42", "21")
-          └─TableReader	0.00	root		data:Selection
-            └─Selection	0.00	cop[tikv]		gt(tpch.customer.c_acctbal, NULL)
-              └─TableFullScan	7500000.00	cop[tikv]	table:customer	keep order:false
+        └─TableReader(Probe)	0.00	root		data:Selection
+          └─Selection	0.00	cop[tikv]		gt(tpch.customer.c_acctbal, NULL), in(substring(tpch.customer.c_phone, 1, 2), "20", "40", "22", "30", "39", "42", "21")
+            └─TableFullScan	7500000.00	cop[tikv]	table:customer	keep order:false

expression/expression.go

@@ -966,7 +966,7 @@ func scalarExprSupportedByTiKV(sf *ScalarFunction) bool {
 
 		// string functions.
 		ast.Length, ast.BitLength, ast.Concat, ast.ConcatWS /*ast.Locate,*/, ast.Replace, ast.ASCII, ast.Hex,
-		ast.Reverse, ast.LTrim, ast.RTrim /*ast.Left,*/, ast.Strcmp, ast.Space, ast.Elt, ast.Field,
+		ast.Reverse, ast.LTrim, ast.RTrim /*ast.Left,*/, ast.Strcmp, ast.Space, ast.Elt, ast.Field, ast.Substring,
 		InternalFuncFromBinary, InternalFuncToBinary,
 
 		// json functions.