[sql] SQLite patch to implement "smart" auto-vacuum.

third_party/sqlite/src/src/btree.c

 /*
 ** 2004 April 6
 **
 ** The author disclaims copyright to this source code.  In place of
 ** a legal notice, here is a blessing:
 **
 **    May you do good and not evil.
 **    May you find forgiveness for yourself and forgive others.
 **    May you share freely, never taking more than you give.
 **
@@ skipping 2794 matching lines @@
   rc = (
     (!p->pBt->autoVacuum)?BTREE_AUTOVACUUM_NONE:
     (!p->pBt->incrVacuum)?BTREE_AUTOVACUUM_FULL:
     BTREE_AUTOVACUUM_INCR
   );
   sqlite3BtreeLeave(p);
   return rc;
 #endif
 }
 
+/*
+** Change the 'auto-vacuum-slack-pages' property of the database. If auto vacuum
+** is enabled, this is the number of chunks of slack to allow before
+** automatically running an incremental vacuum.
+*/
+int sqlite3BtreeSetAutoVacuumSlackPages(Btree *p, int autoVacuumSlack){
+#ifdef SQLITE_OMIT_AUTOVACUUM
+  return SQLITE_READONLY;
+#else
+  BtShared *pBt = p->pBt;
+  int rc = SQLITE_OK;
+  u8 avs = (u8)autoVacuumSlack;
+  if( autoVacuumSlack>avs ){
+    avs = 0xFF;
+  }
+
+  sqlite3BtreeEnter(p);
+  pBt->autoVacuumSlack = avs;
+  sqlite3BtreeLeave(p);
+  return rc;
+#endif
+}
+
+/*
+** Return the value of the 'auto-vacuum-slack-pages' property.
+*/
+int sqlite3BtreeGetAutoVacuumSlackPages(Btree *p){
+#ifdef SQLITE_OMIT_AUTOVACUUM
+  return 0;
+#else
+  int rc = 0;
+  sqlite3BtreeEnter(p);
+  if( p->pBt->autoVacuum!=0 ){
+    rc = p->pBt->autoVacuumSlack;
+  }
+  sqlite3BtreeLeave(p);
+  return rc;
+#endif
+}
+
 
 /*
 ** Get a reference to pPage1 of the database file.  This will
 ** also acquire a readlock on that file.
 **
 ** SQLITE_OK is returned on success.  If the file is not a
 ** well-formed database file, then SQLITE_CORRUPT is returned.
 ** SQLITE_BUSY is returned if the database is locked.  SQLITE_NOMEM
 ** is returned if we run out of memory. 
 */
@@ skipping 821 matching lines @@
 ** This routine is called prior to sqlite3PagerCommit when a transaction
 ** is committed for an auto-vacuum database.
 **
 ** If SQLITE_OK is returned, then *pnTrunc is set to the number of pages
 ** the database file should be truncated to during the commit process. 
 ** i.e. the database has been reorganized so that only the first *pnTrunc
 ** pages are in use.
 */
 static int autoVacuumCommit(BtShared *pBt){
   int rc = SQLITE_OK;
+  int bShouldVacuum = pBt->autoVacuum && !pBt->incrVacuum;
   Pager *pPager = pBt->pPager;
   VVA_ONLY( int nRef = sqlite3PagerRefcount(pPager); )
 
   assert( sqlite3_mutex_held(pBt->mutex) );
   invalidateAllOverflowCache(pBt);
   assert(pBt->autoVacuum);
-  if( !pBt->incrVacuum ){
+  if( bShouldVacuum && pBt->autoVacuumSlack ){
+    Pgno nOrig;        /* Database size before freeing */
+    Pgno nFree;        /* Number of pages on the freelist initially */
+
+    nOrig = btreePagecount(pBt);
+    nFree = get4byte(&pBt->pPage1->aData[36]);
+    bShouldVacuum =
+        (nOrig-nFree-1)/pBt->autoVacuumSlack < (nOrig-1)/pBt->autoVacuumSlack;
+    /* TODO: When integrating this test with the following code, contrive to
+    ** trim to the integral chunk boundary, rather than trimming the entire free
+    ** list.
+    */
+  }
+  if( bShouldVacuum ){
     Pgno nFin;         /* Number of pages in database after autovacuuming */
     Pgno nFree;        /* Number of pages on the freelist initially */
     Pgno iFree;        /* The next page to be freed */
     Pgno nOrig;        /* Database size before freeing */
 
     nOrig = btreePagecount(pBt);
     if( PTRMAP_ISPAGE(pBt, nOrig) || nOrig==PENDING_BYTE_PAGE(pBt) ){
       /* It is not possible to create a database for which the final page
       ** is either a pointer-map page or the pending-byte page. If one
       ** is encountered, this indicates corruption.
@@ skipping 6085 matching lines @@
 /*
 ** Return the number of connections to the BtShared object accessed by
 ** the Btree handle passed as the only argument. For private caches 
 ** this is always 1. For shared caches it may be 1 or greater.
 */
 int sqlite3BtreeConnectionCount(Btree *p){
   testcase( p->sharable );
   return p->pBt->nRef;
 }
 #endif