From: drh <drh@noemail.net>
Date: Wed, 16 Jan 2013 00:46:09 +0000 (+0000)
Subject: Improvements to query planning for joins:  Avoid unnecessary calls to
X-Git-Tag: version-3.7.16~72
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=3bd5ab863810d4d8397e84742db1d6349e754406;p=thirdparty%2Fsqlite.git

Improvements to query planning for joins:  Avoid unnecessary calls to
"optimal scan" checks in cases where table reordering is not possible.
Make sure optimal scan checks are carried out for CROSS JOINs and LEFT
JOINs.

FossilOrigin-Name: d5ebb7877885839e93eee3b322624d4c4215c1c4
---

diff --git a/manifest b/manifest
index b7881441e1..a878fe5539 100644
--- a/manifest
+++ b/manifest
@@ -1,5 +1,5 @@
-C Fix\sa\smissing\sword\sin\sa\scomment.\s\sEnhance\sthe\s"wheretrace"\sdebugging\soutput\nto\sshow\sthe\sestimated\scost\sof\seach\stable\soption\swhile\splanning\sthe\sjoin\sorder.
-D 2013-01-15T18:49:07.627
+C Improvements\sto\squery\splanning\sfor\sjoins:\s\sAvoid\sunnecessary\scalls\sto\n"optimal\sscan"\schecks\sin\scases\swhere\stable\sreordering\sis\snot\spossible.\nMake\ssure\soptimal\sscan\schecks\sare\scarried\sout\sfor\sCROSS\sJOINs\sand\sLEFT\nJOINs.
+D 2013-01-16T00:46:09.175
 F Makefile.arm-wince-mingw32ce-gcc d6df77f1f48d690bd73162294bbba7f59507c72f
 F Makefile.in a48faa9e7dd7d556d84f5456eabe5825dd8a6282
 F Makefile.linux-gcc 91d710bdc4998cb015f39edf3cb314ec4f4d7e23
@@ -252,7 +252,7 @@ F src/vtab.c b05e5f1f4902461ba9f5fc49bb7eb7c3a0741a83
 F src/wal.c f5c7b5027d0ed0e9bc9afeb4a3a8dfea762ec7d2
 F src/wal.h 29c197540b19044e6cd73487017e5e47a1d3dac6
 F src/walker.c 3d75ba73de15e0f8cd0737643badbeb0e002f07b
-F src/where.c b2a827f2b3fa23a3245a4e6093827e359c1e8054
+F src/where.c d48a57d8afd97c51f1b772ebd72431a43a0e48b3
 F test/8_3_names.test 631ea964a3edb091cf73c3b540f6bcfdb36ce823
 F test/aggerror.test a867e273ef9e3d7919f03ef4f0e8c0d2767944f2
 F test/aggnested.test 45c0201e28045ad38a530b5a144b73cd4aa2cfd6
@@ -1033,7 +1033,7 @@ F tool/vdbe-compress.tcl f12c884766bd14277f4fcedcae07078011717381
 F tool/warnings-clang.sh f6aa929dc20ef1f856af04a730772f59283631d4
 F tool/warnings.sh fbc018d67fd7395f440c28f33ef0f94420226381
 F tool/win/sqlite.vsix 97894c2790eda7b5bce3cc79cb2a8ec2fde9b3ac
-P 04507c176330a06b09dcafa35ec0ca7498f5ace7
-R f5ddb099282326d8029eed3fa99d47e6
+P ac4e119a87497f2e422ff1cb711112ed8594bfa9
+R 604db48c0336d28f8c2561951db4000b
 U drh
-Z f7bf9c2b2fa73cf7872c5f7a932db154
+Z 14d78037173bf002814299854e4b798b
diff --git a/manifest.uuid b/manifest.uuid
index a55b102705..536565f198 100644
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-ac4e119a87497f2e422ff1cb711112ed8594bfa9
\ No newline at end of file
+d5ebb7877885839e93eee3b322624d4c4215c1c4
\ No newline at end of file
diff --git a/src/where.c b/src/where.c
index 84d79c811d..ba39fe9a5d 100644
--- a/src/where.c
+++ b/src/where.c
@@ -5072,6 +5072,7 @@ WhereInfo *sqlite3WhereBegin(
     int bestJ = -1;             /* The value of j */
     Bitmask m;                  /* Bitmask value for j or bestJ */
     int isOptimal;              /* Iterator for optimal/non-optimal search */
+    int ckOptimal;              /* Do the optimal scan check */
     int nUnconstrained;         /* Number tables without INDEXED BY */
     Bitmask notIndexed;         /* Mask of tables that cannot use an index */
 
@@ -5122,17 +5123,41 @@ WhereInfo *sqlite3WhereBegin(
     */
     nUnconstrained = 0;
     notIndexed = 0;
-    for(isOptimal=(iFrom<nTabList-1); isOptimal>=0 && bestJ<0; isOptimal--){
+
+    /* The optimal scan check only occurs if there are two or more tables
+    ** available to be reordered */
+    if( iFrom==nTabList-1 ){
+      ckOptimal = 0;  /* Common case of just one table in the FROM clause */
+    }else{
+      ckOptimal = -1;
       for(j=iFrom, sWBI.pSrc=&pTabList->a[j]; j<nTabList; j++, sWBI.pSrc++){
-        int doNotReorder;    /* True if this table should not be reordered */
-  
-        doNotReorder =  (sWBI.pSrc->jointype & (JT_LEFT|JT_CROSS))!=0;
-        if( j!=iFrom && doNotReorder ) break;
         m = getMask(pMaskSet, sWBI.pSrc->iCursor);
         if( (m & sWBI.notValid)==0 ){
           if( j==iFrom ) iFrom++;
           continue;
         }
+        if( j>iFrom && (sWBI.pSrc->jointype & (JT_LEFT|JT_CROSS))!=0 ) break;
+        if( ++ckOptimal ) break;
+        if( (sWBI.pSrc->jointype & JT_LEFT)!=0 ) break;
+      }
+    }
+    assert( ckOptimal==0 || ckOptimal==1 );
+
+    for(isOptimal=ckOptimal; isOptimal>=0 && bestJ<0; isOptimal--){
+      for(j=iFrom, sWBI.pSrc=&pTabList->a[j]; j<nTabList; j++, sWBI.pSrc++){
+        if( j>iFrom && (sWBI.pSrc->jointype & (JT_LEFT|JT_CROSS))!=0 ){
+          /* This break and one like it in the ckOptimal computation loop
+          ** above prevent table reordering across LEFT and CROSS JOINs.
+          ** The LEFT JOIN case is necessary for correctness.  The prohibition
+          ** against reordering across a CROSS JOIN is an SQLite feature that
+          ** allows the developer to control table reordering */
+          break;
+        }
+        m = getMask(pMaskSet, sWBI.pSrc->iCursor);
+        if( (m & sWBI.notValid)==0 ){
+          assert( j>iFrom );
+          continue;
+        }
         sWBI.notReady = (isOptimal ? m : sWBI.notValid);
         if( sWBI.pSrc->pIndex==0 ) nUnconstrained++;
   
@@ -5161,7 +5186,7 @@ WhereInfo *sqlite3WhereBegin(
         }
         if( isOptimal ){
           pWInfo->a[j].rOptCost = sWBI.cost.rCost;
-        }else if( iFrom<nTabList-1 ){
+        }else if( ckOptimal ){
           /* If two or more tables have nearly the same outer loop cost, but
           ** very different inner loop (optimal) cost, we want to choose
           ** for the outer loop that table which benefits the least from
@@ -5207,11 +5232,19 @@ WhereInfo *sqlite3WhereBegin(
           bestPlan = sWBI.cost;
           bestJ = j;
         }
-        if( doNotReorder ) break;
+
+        /* In a join like "w JOIN x LEFT JOIN y JOIN z"  make sure that
+        ** table y (and not table z) is always the next inner loop inside
+        ** of table x. */
+        if( (sWBI.pSrc->jointype & JT_LEFT)!=0 ) break;
       }
     }
     assert( bestJ>=0 );
     assert( sWBI.notValid & getMask(pMaskSet, pTabList->a[bestJ].iCursor) );
+    assert( bestJ==iFrom || (pTabList->a[iFrom].jointype & JT_LEFT)==0 );
+    testcase( bestJ>iFrom && (pTabList->a[iFrom].jointype & JT_CROSS)!=0 );
+    testcase( bestJ>iFrom && bestJ<nTabList-1
+                          && (pTabList->a[bestJ+1].jointype & JT_LEFT)!=0 );
     WHERETRACE(("*** Optimizer selects table %d (%s) for loop %d with:\n"
                 "    cost=%.1f, nRow=%.1f, nOBSat=%d, wsFlags=0x%08x\n",
                 bestJ, pTabList->a[bestJ].pTab->zName,