add some comment

planner/core/exhaust_physical_plans.go

@@ -757,6 +757,12 @@ loopOtherConds:
 	return lastColAccesses
 }
 
+// removeUselessEqAndInFunc removes the useless eq/in conditions. It's designed for the following case:
+//   t1 join t2 on t1.a=t2.a and t1.c=t2.c where t1.b > t2.b-10 and t1.b < t2.b+10 there's index(a, b, c) on t1.
+//   In this case the curIdxOff2KeyOff is [0 -1 1] and the notKeyEqAndIn is [].
+//   It's clearly that the column c cannot be used to access data. So we need to remove it and reset the IdxOff2KeyOff to
+//   [0 -1 -1].
+//   So that we can use t1.a=t2.a and t1.b > t2.b-10 and t1.b < t2.b+10 to build ranges then access data.
 func (ijHelper *indexJoinBuildHelper) removeUselessEqAndInFunc(
 	idxCols []*expression.Column,
 	notKeyEqAndIn []expression.Expression) (