Improved presentation on the new code that prevents unnecessary writes to

expressions on indexes during an UPDATE when the expression does not reference
any of the columns that are changing.

FossilOrigin-Name: c9f045295c4577752b0847ff2027b44661e6cb15bb08b942ccb3a0ef396f3dec

manifest

@@ -1,5 +1,5 @@
-C Increase\sthe\sversion\snumber\sto\s3.26.0\sas\swe\sstart\sthe\snext\sdevelopment\scycle.
-D 2018-09-15T21:43:14.220
+C Improved\spresentation\son\sthe\snew\scode\sthat\sprevents\sunnecessary\swrites\sto\nexpressions\son\sindexes\sduring\san\sUPDATE\swhen\sthe\sexpression\sdoes\snot\sreference\nany\sof\sthe\scolumns\sthat\sare\schanging.
+D 2018-09-16T15:01:25.994
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F Makefile.in 6b650013511fd9d8b094203ac268af9220d292cc7d4e1bc9fbca15aacd8c7995
@@ -567,7 +567,7 @@ F src/threads.c 4ae07fa022a3dc7c5beb373cf744a85d3c5c6c3c
 F src/tokenize.c 9f55961518f77793edd56eee860ecf035d4370ebbb0726ad2f6cada6637fd16b
 F src/treeview.c e7a7f90552bb418533cdd0309b5eb71d4effa50165b880fc8c2001e613577e5f
 F src/trigger.c d3d78568f37fb2e6cdcc2d1e7b60156f15b0b600adec55b83c5d42f6cad250bd
-F src/update.c deb215e3532be46e1e2b6aa38ed9e3eb96e42ebc4cbe7a498cd365788220a432
+F src/update.c 74feccd67570dcb731b6b6ee9960710a5e8a59c8403f2fa88f5eb82dbb8e36fa
 F src/upsert.c 0dd81b40206841814d46942a7337786932475f085716042d0cb2fc7791bf8ca4
 F src/utf.c 810fbfebe12359f10bc2a011520a6e10879ab2a163bcb26c74768eab82ea62a5
 F src/util.c d9eb0a6c4aae1b00a7369eadd7ca0bbe946cb4c953b6751aa20d357c2f482157
@@ -1765,7 +1765,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P a71b101635ed28a4c99734dabb20bd65ef1018c1d63ac143b7321cdb0fafa5d7
-R f789e4a5b00a14bd183ffb1db488fa1d
+P 885f0f8252aae776a86c64bcc7da582f0ed58f2caae8ebff810a83ca339da820
+R 1d15143421504697e5d8ab90a5ba9295
 U drh
-Z 95fb510f742b1570e7d262774a3b689f
+Z 8e73db792f1c5d12613e11c54c1867c6

manifest.uuid

@@ -1 +1 @@
-885f0f8252aae776a86c64bcc7da582f0ed58f2caae8ebff810a83ca339da820
+c9f045295c4577752b0847ff2027b44661e6cb15bb08b942ccb3a0ef396f3dec

src/update.c

@@ -84,18 +84,22 @@ void sqlite3ColumnDefault(Vdbe *v, Table *pTab, int i, int iReg){
 ** columns defined by aXRef and chngRowid.  Return true if it does
 ** and false if not.
 **
-** The iCol-th column of pIdx will be an expression.
-**
 ** aXRef[j] will be non-negative if column j of the original table is
 ** being updated.  chngRowid will be true if the rowid of the table is
 ** being updated.
 */
-static int indexExprRefsUpdatedColumn(
-  Index *pIdx,      /* The index containing the expression to analyze */
-  int iCol,         /* Which column of the index is the expression */
+static int indexColumnIsBeingUpdated(
+  Index *pIdx,      /* The index to check */
+  int iCol,         /* Which column of the index to check */
   int *aXRef,       /* aXRef[j]>=0 if column j is being updated */
   int chngRowid     /* true if the rowid is being updated */
 ){
+  i16 iIdxCol = pIdx->aiColumn[iCol];
+  if( iIdxCol>=0 ){
+    return aXRef[iIdxCol]>=0;
+  }
+  if( iIdxCol==XN_ROWID ) return 1;
+  assert( iIdxCol==XN_EXPR );
   assert( pIdx->aColExpr!=0 );
   assert( pIdx->aColExpr->a[iCol].pExpr!=0 );
   return sqlite3ExprReferencesUpdatedColumn(pIdx->aColExpr->a[iCol].pExpr,
@@ -334,12 +338,7 @@ void sqlite3Update(
     }else{
       reg = 0;
       for(i=0; i<pIdx->nKeyCol; i++){
-        i16 iIdxCol = pIdx->aiColumn[i];
-        if( (iIdxCol>=0 && aXRef[iIdxCol]>=0)
-         || iIdxCol==XN_ROWID
-         || (iIdxCol==XN_EXPR
-             && indexExprRefsUpdatedColumn(pIdx,i,aXRef,chngRowid))
-        ){
+        if( indexColumnIsBeingUpdated(pIdx, i, aXRef, chngRowid) ){
           reg = ++pParse->nMem;
           pParse->nMem += pIdx->nColumn;
           if( (onError==OE_Replace)