Work toward reducing the incremental size of an ExprList object to 24-byte

per entry, from 32-bytes (on a 64-bit machine).  This helps the new
mini-lookaside allocator to run better by avoiding excessive reallocs.
The current change mostly works, but still has a few loose ends to tie up.
This check-in is merely a snapshot to save my work.

FossilOrigin-Name: fdda76cfb01bf2b19522ac4558b443634d28a69b0828677c42682b645eae1f3b

4 files changed, 40 insertions, 28 deletions

diff --git a/src/expr.c b/src/expr.c
index f9c57baa9..0ca1d0407 100644
--- a/src/expr.c
+++ b/src/expr.c
@@ -1437,11 +1437,10 @@ ExprList *sqlite3ExprListDup(sqlite3 *db, ExprList *p, int flags){
       }
     }
     pItem->zEName = sqlite3DbStrDup(db, pOldItem->zEName);
-    pItem->zSpan = sqlite3DbStrDup(db, pOldItem->zSpan);
     pItem->sortFlags = pOldItem->sortFlags;
+    pItem->eEName = pOldItem->eEName;
     pItem->done = 0;
     pItem->bNulls = pOldItem->bNulls;
-    pItem->bSpanIsTab = pOldItem->bSpanIsTab;
     pItem->bSorterRef = pOldItem->bSorterRef;
     pItem->u = pOldItem->u;
   }
@@ -1773,8 +1772,10 @@ void sqlite3ExprListSetSpan(
   if( pList ){
     struct ExprList_item *pItem = &pList->a[pList->nExpr-1];
     assert( pList->nExpr>0 );
-    sqlite3DbFree(db, pItem->zSpan);
-    pItem->zSpan = sqlite3DbSpanDup(db, zStart, zEnd);
+    if( pItem->zEName==0 ){
+      pItem->zEName = sqlite3DbSpanDup(db, zStart, zEnd);
+      pItem->eEName = ENAME_SPAN;
+    }
   }
 }
 
@@ -1805,7 +1806,6 @@ static SQLITE_NOINLINE void exprListDeleteNN(sqlite3 *db, ExprList *pList){
   do{
     sqlite3ExprDelete(db, pItem->pExpr);
     sqlite3DbFree(db, pItem->zEName);
-    sqlite3DbFree(db, pItem->zSpan);
     pItem++;
   }while( --i>0 );
   sqlite3DbFreeNN(db, pList);
diff --git a/src/resolve.c b/src/resolve.c
index b3c331535..cce921031 100644
--- a/src/resolve.c
+++ b/src/resolve.c
@@ -267,7 +267,7 @@ static int lookupName(
           int hit = 0;
           pEList = pItem->pSelect->pEList;
           for(j=0; j<pEList->nExpr; j++){
-            if( sqlite3MatchSpanName(pEList->a[j].zSpan, zCol, zTab, zDb) ){
+            if( sqlite3MatchSpanName(pEList->a[j].zEName, zCol, zTab, zDb) ){
               cnt++;
               cntTab = 2;
               pMatch = pItem;
diff --git a/src/select.c b/src/select.c
index b99b1459b..57bf685ed 100644
--- a/src/select.c
+++ b/src/select.c
@@ -1539,7 +1539,7 @@ static void generateSortTail(
         iRead = iCol--;
       }
       sqlite3VdbeAddOp3(v, OP_Column, iSortTab, iRead, regRow+i);
-      VdbeComment((v, "%s", aOutEx[i].zEName?aOutEx[i].zEName:aOutEx[i].zSpan));
+      VdbeComment((v, "%s", aOutEx[i].zEName));
     }
   }
   switch( eDest ){
@@ -1873,7 +1873,7 @@ static void generateColumnNames(
     assert( p!=0 );
     assert( p->op!=TK_AGG_COLUMN );  /* Agg processing has not run yet */
     assert( p->op!=TK_COLUMN || p->y.pTab!=0 ); /* Covering idx not yet coded */
-    if( pEList->a[i].zEName ){
+    if( pEList->a[i].zEName && pEList->a[i].eEName==ENAME_NAME ){
       /* An AS clause always takes first priority */
       char *zName = pEList->a[i].zEName;
       sqlite3VdbeSetColName(v, i, COLNAME_NAME, zName, SQLITE_TRANSIENT);
@@ -1897,7 +1897,7 @@ static void generateColumnNames(
         sqlite3VdbeSetColName(v, i, COLNAME_NAME, zCol, SQLITE_TRANSIENT);
       }
     }else{
-      const char *z = pEList->a[i].zSpan;
+      const char *z = pEList->a[i].zEName;
       z = z==0 ? sqlite3MPrintf(db, "column%d", i+1) : sqlite3DbStrDup(db, z);
       sqlite3VdbeSetColName(v, i, COLNAME_NAME, z, SQLITE_DYNAMIC);
     }
@@ -1959,7 +1959,7 @@ int sqlite3ColumnsFromExprList(
   for(i=0, pCol=aCol; i<nCol && !db->mallocFailed; i++, pCol++){
     /* Get an appropriate name for the column
     */
-    if( (zName = pEList->a[i].zEName)!=0 ){
+    if( (zName = pEList->a[i].zEName)!=0 && pEList->a[i].eEName==ENAME_NAME ){
       /* If the column contains an "AS <name>" phrase, use <name> as the name */
     }else{
       Expr *pColExpr = sqlite3ExprSkipCollateAndLikely(pEList->a[i].pExpr);
@@ -1979,7 +1979,7 @@ int sqlite3ColumnsFromExprList(
         zName = pColExpr->u.zToken;
       }else{
         /* Use the original text of the column expression as its name */
-        zName = pEList->a[i].zSpan;
+        zName = pEList->a[i].zEName;
       }
     }
     if( zName ){
@@ -5004,9 +5004,8 @@ static int selectExpander(Walker *pWalker, Select *p){
         pNew = sqlite3ExprListAppend(pParse, pNew, a[k].pExpr);
         if( pNew ){
           pNew->a[pNew->nExpr-1].zEName = a[k].zEName;
-          pNew->a[pNew->nExpr-1].zSpan = a[k].zSpan;
+          pNew->a[pNew->nExpr-1].eEName = a[k].eEName;
           a[k].zEName = 0;
-          a[k].zSpan = 0;
         }
         a[k].pExpr = 0;
       }else{
@@ -5045,7 +5044,7 @@ static int selectExpander(Walker *pWalker, Select *p){
 
             assert( zName );
             if( zTName && pSub
-             && sqlite3MatchSpanName(pSub->pEList->a[j].zSpan, 0, zTName, 0)==0
+             && sqlite3MatchSpanName(pSub->pEList->a[j].zEName, 0, zTName, 0)==0
             ){
               continue;
             }
@@ -5098,15 +5097,16 @@ static int selectExpander(Walker *pWalker, Select *p){
             sqlite3ExprListSetName(pParse, pNew, &sColname, 0);
             if( pNew && (p->selFlags & SF_NestedFrom)!=0 ){
               struct ExprList_item *pX = &pNew->a[pNew->nExpr-1];
+              sqlite3DbFree(db, pX->zEName);
               if( pSub ){
-                pX->zSpan = sqlite3DbStrDup(db, pSub->pEList->a[j].zSpan);
-                testcase( pX->zSpan==0 );
+                pX->zEName = sqlite3DbStrDup(db, pSub->pEList->a[j].zEName);
+                testcase( pX->zEName==0 );
               }else{
-                pX->zSpan = sqlite3MPrintf(db, "%s.%s.%s",
+                pX->zEName = sqlite3MPrintf(db, "%s.%s.%s",
                                            zSchemaName, zTabName, zColname);
-                testcase( pX->zSpan==0 );
+                testcase( pX->zEName==0 );
               }
-              pX->bSpanIsTab = 1;
+              pX->eEName = ENAME_TAB;
             }
             sqlite3DbFree(db, zToFree);
           }
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index 3de659f79..7425b2988 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -2650,23 +2650,28 @@ struct Expr {
 ** also be used as the argument to a function, in which case the a.zName
 ** field is not used.
 **
-** By default the Expr.zSpan field holds a human-readable description of
-** the expression that is used in the generation of error messages and
-** column labels.  In this case, Expr.zSpan is typically the text of a
-** column expression as it exists in a SELECT statement.  However, if
-** the bSpanIsTab flag is set, then zSpan is overloaded to mean the name
-** of the result column in the form: DATABASE.TABLE.COLUMN.  This later
-** form is used for name resolution with nested FROM clauses.
+** In order to try to keep memory usage down, the Expr.a.zEName field
+** is used for multiple purposes:
+**
+**     bNameIsTab    bNameIsSpan        Usage
+**     ----------    -----------        -------------------------
+**       false          false           (1) the AS of result set column
+**                                      (2) COLUMN= of an UPDATE
+**
+**       true           false           DB.TABLE.NAME used to resolve names
+**                                      of subqueries
+**
+**       false          true            Text of the original result set
+**                                      expression.
 */
 struct ExprList {
   int nExpr;             /* Number of expressions on the list */
   struct ExprList_item { /* For each expression in the list */
     Expr *pExpr;            /* The parse tree for this expression */
     char *zEName;           /* Token associated with this expression */
-    char *zSpan;            /* Original text of the expression */
     u8 sortFlags;           /* Mask of KEYINFO_ORDER_* flags */
+    unsigned eEName :2;     /* Meaning of zEName */
     unsigned done :1;       /* A flag to indicate when processing is finished */
-    unsigned bSpanIsTab :1; /* zSpan holds DB.TABLE.COLUMN */
     unsigned reusable :1;   /* Constant expression is reusable */
     unsigned bSorterRef :1; /* Defer evaluation until after sorting */
     unsigned bNulls: 1;     /* True if explicit "NULLS FIRST/LAST" */
@@ -2681,6 +2686,13 @@ struct ExprList {
 };
 
 /*
+** Allowed values for Expr.a.eEName
+*/
+#define ENAME_NAME  0       /* The AS clause of a result set */
+#define ENAME_SPAN  1       /* Complete text of the result set expression */
+#define ENAME_TAB   2       /* "DB.TABLE.NAME" for the result set */
+
+/*
 ** An instance of this structure can hold a simple list of identifiers,
 ** such as the list "a,b,c" in the following statements:
 **