Additional comments on the sqlite3ExprIsConstantOrGroupBy() routine. No

code changes.

FossilOrigin-Name: 8424492eac506866d2918e5fe03c8f65fef960215d56012a3b52ed42789ed35a

manifest

@@ -1,5 +1,5 @@
-C Remove\san\sunnecessary\sbranch.
+C Additional\scomments\son\sthe\ssqlite3ExprIsConstantOrGroupBy()\sroutine.\s\sNo\ncode\schanges.
-D 2017-05-01T19:53:12.478
+D 2017-05-02T16:46:41.947
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-F src/expr.c ed8914c001a24ab25aee965f498c96de577bf2c4a6145022b3688c11c2b29c82
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manifest.uuid

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-a33179596f534067a58d68b77160f11ab13272b29f912d7cbe15ea00bbf03ade
+8424492eac506866d2918e5fe03c8f65fef960215d56012a3b52ed42789ed35a

src/expr.c

@@ -1848,6 +1848,20 @@ static int exprNodeIsConstantOrGroupBy(Walker *pWalker, Expr *pExpr){
 ** Walk the expression tree passed as the first argument. Return non-zero
 ** if the expression consists entirely of constants or copies of terms 
 ** in pGroupBy that sort with the BINARY collation sequence.
+**
+** This routine is used to determine if a term of the HAVING clause can
+** be promoted into the WHERE clause.  In order for such a promotion to work,
+** the value of the HAVING clause term must be the same for all members of
+** a "group".  The requirement that the GROUP BY term must be BINARY
+** assumes that no other collating sequence will have a finer-grained
+** grouping than binary.  In other words (A=B COLLATE binary) implies
+** A=B in every other collating sequence.  The requirement that the
+** GROUP BY be BINARY is stricter than necessary.  It would also work
+** to promote HAVING clauses that use the same alternative collating
+** sequence as the GROUP BY term, but that is much harder to check,
+** alternative collating sequences are uncommon, and this is only an
+** optimization, so we take the easy way out and simply require the
+** GROUP BY to use the BINARY collating sequence.
 */
 int sqlite3ExprIsConstantOrGroupBy(Parse *pParse, Expr *p, ExprList *pGroupBy){
   Walker w;