1 files changed, 17 insertions, 5 deletions
diff --git a/src/select.c b/src/select.c
index b13aae1f9..9c2bf3b93 100644
--- a/src/select.c
+++ b/src/select.c
@@ -5060,6 +5060,18 @@ static int pushDownWindowCheck(Parse *pParse, Select *pSubq, Expr *pExpr){
 ** The hope is that the terms added to the inner query will make it more
 ** efficient.
 **
+** NAME AMBIGUITY
+**
+** This optimization is called the "WHERE-clause push-down optimization".
+**
+** Do not confuse this optimization with another unrelated optimization
+** with a similar name:  The "MySQL push-down optimization" causes WHERE
+** clause terms that can be evaluated using only the index and without
+** reference to the table are run first, so that if they are false,
+** unnecessary table seeks are avoided.
+**
+** RULES
+**
 ** Do not attempt this optimization if:
 **
 **   (1) (** This restriction was removed on 2017-09-29.  We used to
@@ -5125,10 +5137,10 @@ static int pushDownWindowCheck(Parse *pParse, Select *pSubq, Expr *pExpr){
 **       (9c) There is a RIGHT JOIN (or FULL JOIN) in between the ON/USING
 **            clause and the subquery.
 **
-**       Without this restriction, the push-down optimization might move
-**       the ON/USING filter expression from the left side of a RIGHT JOIN
-**       over to the right side, which leads to incorrect answers.  See
-**       also restriction (6) in sqlite3ExprIsSingleTableConstraint().
+**       Without this restriction, the WHERE-clause push-down optimization
+**       might move the ON/USING filter expression from the left side of a
+**       RIGHT JOIN over to the right side, which leads to incorrect answers.
+**       See also restriction (6) in sqlite3ExprIsSingleTableConstraint().
 **
 **  (10) The inner query is not the right-hand table of a RIGHT JOIN.
 **
@@ -7689,7 +7701,7 @@ int sqlite3Select(
 #endif
       assert( pItem->pSelect && (pItem->pSelect->selFlags & SF_PushDown)!=0 );
     }else{
-      TREETRACE(0x4000,pParse,p,("Push-down not possible\n"));
+      TREETRACE(0x4000,pParse,p,("WHERE-lcause push-down not possible\n"));
     }
 
     /* Convert unused result columns of the subquery into simple NULL