In the BTree subsystem, when using pages from the freelist, attempt to select

pages close to related pages in order to keep data structures near each other
in the database file.  This improves access speed in some circumstances. (CVS 667)

FossilOrigin-Name: fd7e41f0eed80fb1c7e18eb84834ec3cea74a649

manifest

@@ -1,5 +1,5 @@
-C Make\sthe\sBTree\sbalance()\sroutine\sa\slittle\sfaster\sby\sreusing\sdatabase\r\npages\slocally\srather\sthan\sfreeing\sand\sreallocating\sthem.\s(CVS\s666)
-D 2002-07-08T02:16:38
+C In\sthe\sBTree\ssubsystem,\swhen\susing\spages\sfrom\sthe\sfreelist,\sattempt\sto\sselect\npages\sclose\sto\srelated\spages\sin\sorder\sto\skeep\sdata\sstructures\snear\seach\sother\nin\sthe\sdatabase\sfile.\s\sThis\simproves\saccess\sspeed\sin\ssome\scircumstances.\s(CVS\s667)
+D 2002-07-08T10:59:51
 F Makefile.in 6291a33b87d2a395aafd7646ee1ed562c6f2c28c
 F Makefile.template 4e11752e0b5c7a043ca50af4296ec562857ba495
 F README a4c0ba11354ef6ba0776b400d057c59da47a4cc0
@@ -18,7 +18,7 @@ F publish.sh 1a04b9aa0d9c9661e338268343476ed0851c5778
 F spec.template 238f7db425a78dc1bb7682e56e3834c7270a3f5e
 F sqlite.1 83f4a9d37bdf2b7ef079a82d54eaf2e3509ee6ea
 F src/TODO af7f3cab0228e34149cf98e073aa83d45878e7e6
-F src/btree.c ce999ee3f5c6130aa12529ba4f510679d0a07faa
+F src/btree.c db8cd1bd46cd30a1763c3cc80602571d1b30a329
 F src/btree.h 8abeabfe6e0b1a990b64fa457592a6482f6674f3
 F src/build.c ea4a3bc15d6338294e68100f642edf48e4082403
 F src/delete.c 215492ffcea4262a993e55f3c4a67dc9fea4da9c
@@ -140,7 +140,7 @@ F www/speed.tcl da8afcc1d3ccc5696cfb388a68982bc3d9f7f00f
 F www/sqlite.tcl ae3dcfb077e53833b59d4fcc94d8a12c50a44098
 F www/tclsqlite.tcl 1db15abeb446aad0caf0b95b8b9579720e4ea331
 F www/vdbe.tcl 2013852c27a02a091d39a766bc87cff329f21218
-P 111c78e6835306fcd8b6d22b9ae68dfb9ab4febe
-R 66eadf0358149489c17da262453c92f0
+P 3c2dea4310af491d6cb09856d4bc5236d6dc44ac
+R 6fce7034f8d829ff998a11d6ed601ee8
 U drh
-Z 70877f6880980e12b35ccd8168df5635
+Z 0f3c77c73454501b08d2d78dacfaba6f

manifest.uuid

@@ -1 +1 @@
-3c2dea4310af491d6cb09856d4bc5236d6dc44ac
+fd7e41f0eed80fb1c7e18eb84834ec3cea74a649

src/btree.c

@@ -9,7 +9,7 @@
 **    May you share freely, never taking more than you give.
 **
 *************************************************************************
-** $Id: btree.c,v 1.66 2002/07/08 02:16:38 drh Exp $
+** $Id: btree.c,v 1.67 2002/07/08 10:59:51 drh Exp $
 **
 ** This file implements a external (disk-based) database using BTrees.
 ** For a detailed discussion of BTrees, refer to
@@ -1509,8 +1509,13 @@ int sqliteBtreeNext(BtCursor *pCur, int *pRes){
 ** SQLITE_OK is returned on success.  Any other return value indicates
 ** an error.  *ppPage and *pPgno are undefined in the event of an error.
 ** Do not invoke sqlitepager_unref() on *ppPage if an error is returned.
+**
+** If the "near" parameter is not 0, then a (feeble) effort is made to 
+** locate a page close to the page number "near".  This can be used in an
+** attempt to keep related pages close to each other in the database file,
+** which in turn can make database access faster.
 */
-static int allocatePage(Btree *pBt, MemPage **ppPage, Pgno *pPgno){
+static int allocatePage(Btree *pBt, MemPage **ppPage, Pgno *pPgno, Pgno near){
   PageOne *pPage1 = pBt->page1;
   int rc;
   if( pPage1->freeList ){
@@ -1533,8 +1538,23 @@ static int allocatePage(Btree *pBt, MemPage **ppPage, Pgno *pPgno){
       pPage1->freeList = pOvfl->iNext;
       *ppPage = (MemPage*)pOvfl;
     }else{
+      int closest;
+      if( pInfo->nFree>1 && near>0 ){
+        int i, dist;
+        closest = 0;
+        dist = pInfo->aFree[0] - near;
+        if( dist<0 ) dist = -dist;
+        for(i=1; i<pInfo->nFree; i++){
+          int d2 = pInfo->aFree[i] - near;
+          if( d2<0 ) d2 = -d2;
+          if( d2<dist ) closest = i;
+        }
+      }else{
+        closest = 0;
+      }
       pInfo->nFree--;
-      *pPgno = pInfo->aFree[pInfo->nFree];
+      *pPgno = pInfo->aFree[closest];
+      pInfo->aFree[closest] = pInfo->aFree[pInfo->nFree];
       rc = sqlitepager_get(pBt->pPager, *pPgno, (void**)ppPage);
       sqlitepager_unref(pOvfl);
       if( rc==SQLITE_OK ){
@@ -1660,6 +1680,7 @@ static int fillInCell(
   int nPayload;
   const char *pPayload;
   char *pSpace;
+  Pgno near = 0;
 
   pCell->h.leftChild = 0;
   pCell->h.nKey = nKey & 0xffff;
@@ -1677,9 +1698,11 @@ static int fillInCell(
   pPrior = 0;
   while( nPayload>0 ){
     if( spaceLeft==0 ){
-      rc = allocatePage(pBt, (MemPage**)&pOvfl, pNext);
+      rc = allocatePage(pBt, (MemPage**)&pOvfl, pNext, near);
       if( rc ){
         *pNext = 0;
+      }else{
+        near = *pNext;
       }
       if( pPrior ) sqlitepager_unref(pPrior);
       if( rc ){
@@ -1986,7 +2009,7 @@ static int balance(Btree *pBt, MemPage *pPage, BtCursor *pCur){
     */
     rc = sqlitepager_write(pPage);
     if( rc ) return rc;
-    rc = allocatePage(pBt, &pChild, &pgnoChild);
+    rc = allocatePage(pBt, &pChild, &pgnoChild, sqlitepager_pagenumber(pPage));
     if( rc ) return rc;
     assert( sqlitepager_iswriteable(pChild) );
     copyPage(pChild, pPage);
@@ -2171,7 +2194,7 @@ static int balance(Btree *pBt, MemPage *pPage, BtCursor *pCur){
       apOld[i] = 0;
       sqlitepager_write(apNew[i]);
     }else{
-      rc = allocatePage(pBt, &apNew[i], &pgnoNew[i]);
+      rc = allocatePage(pBt, &apNew[i], &pgnoNew[i], pgnoNew[i-1]);
       if( rc ) goto balance_cleanup;
     }
     nNew++;
@@ -2455,7 +2478,7 @@ int sqliteBtreeCreateTable(Btree *pBt, int *piTable){
   if( pBt->readOnly ){
     return SQLITE_READONLY;
   }
-  rc = allocatePage(pBt, &pRoot, &pgnoRoot);
+  rc = allocatePage(pBt, &pRoot, &pgnoRoot, 0);
   if( rc ) return rc;
   assert( sqlitepager_iswriteable(pRoot) );
   zeroPage(pRoot);