1 files changed, 173 insertions, 56 deletions

diff --git a/src/where.c b/src/where.c
index 0b8e5acea..5cb52b8ad 100644
--- a/src/where.c
+++ b/src/where.c
@@ -2431,8 +2431,9 @@ void sqlite3WhereClausePrint(WhereClause *pWC){
 **     1.002.001         t2.t2xy              2 f 010241 N 2 cost 0,56,31
 */
 void sqlite3WhereLoopPrint(const WhereLoop *p, const WhereClause *pWC){
+  WhereInfo *pWInfo;
   if( pWC ){
-    WhereInfo *pWInfo = pWC->pWInfo;
+    pWInfo = pWC->pWInfo;
     int nb = 1+(pWInfo->pTabList->nSrc+3)/4;
     SrcItem *pItem = pWInfo->pTabList->a + p->iTab;
     Table *pTab = pItem->pSTab;
@@ -2442,6 +2443,7 @@ void sqlite3WhereLoopPrint(const WhereLoop *p, const WhereClause *pWC){
     sqlite3DebugPrintf(" %12s",
                        pItem->zAlias ? pItem->zAlias : pTab->zName);
   }else{
+    pWInfo = 0;
     sqlite3DebugPrintf("%c%2d.%03llx.%03llx %c%d",
          p->cId, p->iTab, p->maskSelf, p->prereq & 0xfff, p->cId, p->iTab);
   }
@@ -2473,7 +2475,12 @@ void sqlite3WhereLoopPrint(const WhereLoop *p, const WhereClause *pWC){
   }else{
     sqlite3DebugPrintf(" f %06x N %d", p->wsFlags, p->nLTerm);
   }
-  sqlite3DebugPrintf(" cost %d,%d,%d\n", p->rSetup, p->rRun, p->nOut);
+  if( pWInfo && pWInfo->bStarUsed && p->rStarDelta!=0 ){
+    sqlite3DebugPrintf(" cost %d,%d,%d delta=%d\n",
+                       p->rSetup, p->rRun, p->nOut, p->rStarDelta);
+  }else{
+    sqlite3DebugPrintf(" cost %d,%d,%d\n", p->rSetup, p->rRun, p->nOut);
+  }
   if( p->nLTerm && (sqlite3WhereTrace & 0x4000)!=0 ){
     int i;
     for(i=0; i<p->nLTerm; i++){
@@ -5441,77 +5448,179 @@ static LogEst whereSortingCost(
 **     18    for star queries
 **     12    otherwise
 **
-** For the purposes of SQLite, a star-query is defined as a query
-** with a large central table that is joined (using an INNER JOIN,
-** not a LEFT JOIN) against four or more smaller tables.  The central
-** table is called the "fact" table.  The smaller tables that get
-** joined are "dimension tables".
+** For the purposes of this heuristic, a star-query is defined as a query
+** with a large central table that is joined using an INNER JOIN,
+** not CROSS or OUTER JOINs, against four or more smaller tables.
+** The central table is called the "fact" table.  The smaller tables
+** that get joined are "dimension tables".  Also, any table that is
+** self-joined cannot be a dimension table; we assume that dimension
+** tables may only be joined against fact tables.
 **
 ** SIDE EFFECT:  (and really the whole point of this subroutine)
 **
-** If pWInfo describes a star-query, then the cost on WhereLoops for the
-** fact table is reduced.  This heuristic helps keep fact tables in
-** outer loops.  Without this heuristic, paths with fact tables in outer
-** loops tend to get pruned by the mxChoice limit on the number of paths,
-** resulting in poor query plans.  The total amount of heuristic cost
-** adjustment is stored in pWInfo->nOutStarDelta and the cost adjustment
-** for each WhereLoop is stored in its rStarDelta field.
+** If pWInfo describes a star-query, then the cost for SCANs of dimension
+** WhereLoops is increased to be slightly larger than the cost of a SCAN
+** in the fact table.  Only SCAN costs are increased.  SEARCH costs are
+** unchanged. This heuristic helps keep fact tables in outer loops. Without
+** this heuristic, paths with fact tables in outer loops tend to get pruned
+** by the mxChoice limit on the number of paths, resulting in poor query
+** plans.  See the starschema1.test test module for examples of queries
+** that need this heuristic to find good query plans.
+**
+** This heuristic can be completely disabled, so that no query is
+** considered a star-query, using SQLITE_TESTCTRL_OPTIMIZATION to
+** disable the SQLITE_StarQuery optimization.  In the CLI, the command
+** to do that is:  ".testctrl opt -starquery".
+**
+** HISTORICAL NOTES:
+**
+** This optimization was first added on 2024-05-09 by check-in 38db9b5c83d.
+** The original optimization reduced the cost and output size estimate for
+** fact tables to help them move to outer loops.  But months later (as people
+** started upgrading) performance regression reports started caming in,
+** including:
+**
+**    forum post b18ef983e68d06d1 (2024-12-21)
+**    forum post 0025389d0860af82 (2025-01-14)
+**    forum post d87570a145599033 (2025-01-17)
+**
+** To address these, the criteria for a star-query was tightened to exclude
+** cases where the fact and dimensions are separated by an outer join, and
+** the affect of star-schema detection was changed to increase the rRun cost
+** on just full table scans of dimension tables, rather than reducing costs
+** in the all access methods of the fact table.
 */
-static int computeMxChoice(WhereInfo *pWInfo, LogEst nRowEst){
+static int computeMxChoice(WhereInfo *pWInfo){
   int nLoop = pWInfo->nLevel;    /* Number of terms in the join */
-  if( nRowEst==0
-   && nLoop>=5 
+  WhereLoop *pWLoop;             /* For looping over WhereLoops */
+
+#ifdef SQLITE_DEBUG
+  /* The star-query detection code below makes use of the following
+  ** properties of the WhereLoop list, so verify them before
+  ** continuing:
+  **    (1)  .maskSelf is the bitmask corresponding to .iTab
+  **    (2)  The WhereLoop list is in ascending .iTab order
+  */
+  for(pWLoop=pWInfo->pLoops; pWLoop; pWLoop=pWLoop->pNextLoop){
+    assert( pWLoop->maskSelf==MASKBIT(pWLoop->iTab) );
+    assert( pWLoop->pNextLoop==0 || pWLoop->iTab<=pWLoop->pNextLoop->iTab );
+  }
+#endif /* SQLITE_DEBUG */
+
+  if( nLoop>=5
+   && !pWInfo->bStarDone
    && OptimizationEnabled(pWInfo->pParse->db, SQLITE_StarQuery)
   ){
-    /* Check to see if we are dealing with a star schema and if so, reduce
-    ** the cost of fact tables relative to dimension tables, as a heuristic
-    ** to help keep the fact tables in outer loops.
+    SrcItem *aFromTabs;    /* All terms of the FROM clause */
+    int iFromIdx;          /* Term of FROM clause is the candidate fact-table */
+    Bitmask m;             /* Bitmask for candidate fact-table */
+    Bitmask mSelfJoin = 0; /* Tables that cannot be dimension tables */       
+    WhereLoop *pStart;     /* Where to start searching for dimension-tables */
+
+    pWInfo->bStarDone = 1; /* Only do this computation once */
+
+    /* Look for fact tables with four or more dimensions where the
+    ** dimension tables are not separately from the fact tables by an outer
+    ** or cross join.  Adjust cost weights if found.
     */
-    int iLoop;                /* Counter over join terms */
-    Bitmask m;                /* Bitmask for current loop */
-    assert( pWInfo->nOutStarDelta==0 );
-    for(iLoop=0, m=1; iLoop<nLoop; iLoop++, m<<=1){
-      WhereLoop *pWLoop;        /* For looping over WhereLoops */
+    assert( !pWInfo->bStarUsed );
+    aFromTabs = pWInfo->pTabList->a;
+    pStart = pWInfo->pLoops;
+    for(iFromIdx=0, m=1; iFromIdx<nLoop; iFromIdx++, m<<=1){
       int nDep = 0;             /* Number of dimension tables */
-      LogEst rDelta;            /* Heuristic cost adjustment */
+      LogEst mxRun;             /* Maximum SCAN cost of a fact table */
       Bitmask mSeen = 0;        /* Mask of dimension tables */
-      for(pWLoop=pWInfo->pLoops; pWLoop; pWLoop=pWLoop->pNextLoop){
-        if( (pWLoop->prereq & m)!=0        /* pWInfo depends on iLoop */
+      SrcItem *pFactTab;        /* The candidate fact table */
+
+      pFactTab = aFromTabs + iFromIdx;
+      if( (pFactTab->fg.jointype & (JT_OUTER|JT_CROSS))!=0 ){
+        /* If the candidate fact-table is the right table of an outer join
+        ** restrict the search for dimension-tables to be tables to the right
+        ** of the fact-table. */
+        if( iFromIdx+4 > nLoop ) break;  /* Impossible to reach nDep>=4 */
+        while( pStart && pStart->iTab<=iFromIdx ){
+          pStart = pStart->pNextLoop;
+        }
+      }
+      for(pWLoop=pStart; pWLoop; pWLoop=pWLoop->pNextLoop){
+        if( (aFromTabs[pWLoop->iTab].fg.jointype & (JT_OUTER|JT_CROSS))!=0 ){
+          /* Fact-tables and dimension-tables cannot be separated by an
+          ** outer join (at least for the definition of fact- and dimension-
+          ** used by this heuristic). */
+          break;
+        }
+        if( (pWLoop->prereq & m)!=0        /* pWInfo depends on iFromIdx */
          && (pWLoop->maskSelf & mSeen)==0  /* pWInfo not already a dependency */
-         && (pWInfo->pTabList->a[pWLoop->iTab].fg.jointype & JT_LEFT)==0
-                                               /* ^- pWInfo isn't a LEFT JOIN */
+         && (pWLoop->maskSelf & mSelfJoin)==0 /* Not a self-join */
         ){
-          nDep++;
-          mSeen |= pWLoop->maskSelf;
+          if( aFromTabs[pWLoop->iTab].pSTab==pFactTab->pSTab ){
+            mSelfJoin |= m;
+          }else{
+            nDep++;
+            mSeen |= pWLoop->maskSelf;
+          }
         }
       }
       if( nDep<=3 ) continue;
-      rDelta = 15*(nDep-3);
-#ifdef WHERETRACE_ENABLED /* 0x4 */
-      if( sqlite3WhereTrace&0x4 ){
-         SrcItem *pItem = pWInfo->pTabList->a + iLoop;
-         sqlite3DebugPrintf(
-             "Fact-table %s(%d): %d dimensions, cost reduced %d\n",
-             pItem->zAlias ? pItem->zAlias : pItem->pSTab->zName, iLoop,
-             nDep, rDelta);
-      }
-#endif
-      if( pWInfo->nOutStarDelta==0 ){
+
+      /* If we reach this point, it means that pFactTab is a fact table
+      ** with four or more dimensions connected by inner joins.  Proceed
+      ** to make cost adjustments. */
+     
+#ifdef WHERETRACE_ENABLED
+      /* Make sure rStarDelta values are initialized */
+      if( !pWInfo->bStarUsed ){
         for(pWLoop=pWInfo->pLoops; pWLoop; pWLoop=pWLoop->pNextLoop){
           pWLoop->rStarDelta = 0;
         }
       }
-      pWInfo->nOutStarDelta += rDelta;
+#endif
+      pWInfo->bStarUsed = 1;
+
+      /* Compute the maximum cost of any WhereLoop for the
+      ** fact table plus one epsilon */
+      mxRun = LOGEST_MIN;
+      for(pWLoop=pStart; pWLoop; pWLoop=pWLoop->pNextLoop){
+        if( pWLoop->iTab<iFromIdx ) continue;
+        if( pWLoop->iTab>iFromIdx ) break;
+        if( pWLoop->rRun>mxRun ) mxRun = pWLoop->rRun;
+      }
+      if( ALWAYS(mxRun<LOGEST_MAX) ) mxRun++;
+
+      /* Increase the cost of table scans for dimension tables to be
+      ** slightly more than the maximum cost of the fact table */
+      for(pWLoop=pStart; pWLoop; pWLoop=pWLoop->pNextLoop){
+        if( (pWLoop->maskSelf & mSeen)==0 ) continue;
+        if( pWLoop->nLTerm ) continue;
+        if( pWLoop->rRun<mxRun ){
+#ifdef WHERETRACE_ENABLED /* 0x80000 */
+          if( sqlite3WhereTrace & 0x80000 ){
+            SrcItem *pDim = aFromTabs + pWLoop->iTab;
+            sqlite3DebugPrintf(
+              "Increase SCAN cost of dimension %s(%d) of fact %s(%d) to %d\n",
+              pDim->zAlias ? pDim->zAlias: pDim->pSTab->zName, pWLoop->iTab,
+              pFactTab->zAlias ? pFactTab->zAlias : pFactTab->pSTab->zName,
+              iFromIdx, mxRun
+            );
+          }
+          pWLoop->rStarDelta = mxRun - pWLoop->rRun;
+#endif /* WHERETRACE_ENABLED */
+          pWLoop->rRun = mxRun;
+        }
+      }
+    }
+#ifdef WHERETRACE_ENABLED /* 0x80000 */
+    if( (sqlite3WhereTrace & 0x80000)!=0 && pWInfo->bStarUsed ){
+      sqlite3DebugPrintf("WhereLoops changed by star-query heuristic:\n");
       for(pWLoop=pWInfo->pLoops; pWLoop; pWLoop=pWLoop->pNextLoop){
-        if( pWLoop->maskSelf==m ){
-          pWLoop->rRun -= rDelta;
-          pWLoop->nOut -= rDelta;
-          pWLoop->rStarDelta = rDelta;
+        if( pWLoop->rStarDelta ){
+          sqlite3WhereLoopPrint(pWLoop, &pWInfo->sWC);
         }
       }
-    }      
+    }
+#endif
   }
-  return pWInfo->nOutStarDelta>0 ? 18 : 12;
+  return pWInfo->bStarUsed ? 18 : 12;
 }
 
 /*
@@ -5586,8 +5695,10 @@ static int wherePathSolver(WhereInfo *pWInfo, LogEst nRowEst){
     mxChoice = 1;
   }else if( nLoop==2 ){
     mxChoice = 5;
+  }else if( pParse->nErr ){
+    mxChoice = 1;
   }else{
-    mxChoice = computeMxChoice(pWInfo, nRowEst);
+    mxChoice = computeMxChoice(pWInfo);
   }
   assert( nLoop<=pWInfo->pTabList->nSrc );
 
@@ -5838,8 +5949,10 @@ static int wherePathSolver(WhereInfo *pWInfo, LogEst nRowEst){
     if( sqlite3WhereTrace & 0x02 ){
       LogEst rMin, rFloor = 0;
       int nDone = 0;
+      int nProgress;
       sqlite3DebugPrintf("---- after round %d ----\n", iLoop);
-      while( nDone<nTo ){
+      do{
+        nProgress = 0;
         rMin = 0x7fff;
         for(ii=0, pTo=aTo; ii<nTo; ii++, pTo++){
           if( pTo->rCost>rFloor && pTo->rCost<rMin ) rMin = pTo->rCost;
@@ -5855,10 +5968,11 @@ static int wherePathSolver(WhereInfo *pWInfo, LogEst nRowEst){
               sqlite3DebugPrintf("\n");
             }
             nDone++;
+            nProgress++;
           }
         }
         rFloor = rMin;
-      }
+      }while( nDone<nTo && nProgress>0 );
     }
 #endif
 
@@ -5952,7 +6066,10 @@ static int wherePathSolver(WhereInfo *pWInfo, LogEst nRowEst){
     }
   }
 
-  pWInfo->nRowOut = pFrom->nRow + pWInfo->nOutStarDelta;
+  pWInfo->nRowOut = pFrom->nRow;
+#ifdef WHERETRACE_ENABLED
+  pWInfo->rTotalCost = pFrom->rCost;
+#endif
 
   /* Free temporary memory and return success */
   sqlite3StackFreeNN(pParse->db, pSpace);
@@ -6350,7 +6467,6 @@ static SQLITE_NOINLINE void whereCheckIfBloomFilterIsUseful(
       }
     }
     nSearch += pLoop->nOut;
-    if( pWInfo->nOutStarDelta ) nSearch += pLoop->rStarDelta;
   }
 }
 
@@ -6833,7 +6949,8 @@ WhereInfo *sqlite3WhereBegin(
   assert( db->mallocFailed==0 );
 #ifdef WHERETRACE_ENABLED
   if( sqlite3WhereTrace ){
-    sqlite3DebugPrintf("---- Solution nRow=%d", pWInfo->nRowOut);
+    sqlite3DebugPrintf("---- Solution cost=%d, nRow=%d",
+                       pWInfo->rTotalCost, pWInfo->nRowOut);
     if( pWInfo->nOBSat>0 ){
       sqlite3DebugPrintf(" ORDERBY=%d,0x%llx", pWInfo->nOBSat, pWInfo->revMask);
     }