Keep track of the optimal number of reserved bytes (by looking at reserve

byte requests in calls to sqlite3BtreeSetPageSize()) and then change the
reserve byte count to the optimal when doing a VACUUM or when using the
backup API.

FossilOrigin-Name: 28c2b726285ea88b334acfd6390a057d2d244838

1 files changed, 16 insertions, 7 deletions

diff --git a/src/btree.c b/src/btree.c
index eb5151351..8957b74c1 100644
--- a/src/btree.c
+++ b/src/btree.c
@@ -2405,6 +2405,9 @@ int sqlite3BtreeSetPageSize(Btree *p, int pageSize, int nReserve, int iFix){
   BtShared *pBt = p->pBt;
   assert( nReserve>=-1 && nReserve<=255 );
   sqlite3BtreeEnter(p);
+#if SQLITE_HAS_CODEC
+  if( nReserve>pBt->optimalReserve ) pBt->optimalReserve = (u8)nReserve;
+#endif
   if( pBt->btsFlags & BTS_PAGESIZE_FIXED ){
     sqlite3BtreeLeave(p);
     return SQLITE_READONLY;
@@ -2434,7 +2437,6 @@ int sqlite3BtreeGetPageSize(Btree *p){
   return p->pBt->pageSize;
 }
 
-#if defined(SQLITE_HAS_CODEC) || defined(SQLITE_DEBUG)
 /*
 ** This function is similar to sqlite3BtreeGetReserve(), except that it
 ** may only be called if it is guaranteed that the b-tree mutex is already
@@ -2447,25 +2449,33 @@ int sqlite3BtreeGetPageSize(Btree *p){
 ** database handle that owns *p, causing undefined behavior.
 */
 int sqlite3BtreeGetReserveNoMutex(Btree *p){
+  int n;
   assert( sqlite3_mutex_held(p->pBt->mutex) );
-  return p->pBt->pageSize - p->pBt->usableSize;
+  n = p->pBt->pageSize - p->pBt->usableSize;
+  return n;
 }
-#endif /* SQLITE_HAS_CODEC || SQLITE_DEBUG */
 
-#if !defined(SQLITE_OMIT_PAGER_PRAGMAS) || !defined(SQLITE_OMIT_VACUUM)
 /*
 ** Return the number of bytes of space at the end of every page that
 ** are intentually left unused.  This is the "reserved" space that is
 ** sometimes used by extensions.
+**
+** If SQLITE_HAS_MUTEX is defined then the number returned is the
+** greater of the current reserved space and the maximum requested
+** reserve space.
 */
-int sqlite3BtreeGetReserve(Btree *p){
+int sqlite3BtreeGetOptimalReserve(Btree *p){
   int n;
   sqlite3BtreeEnter(p);
-  n = p->pBt->pageSize - p->pBt->usableSize;
+  n = sqlite3BtreeGetReserveNoMutex(p);
+#ifdef SQLITE_HAS_CODEC
+  if( n<p->pBt->optimalReserve ) n = p->pBt->optimalReserve;
+#endif
   sqlite3BtreeLeave(p);
   return n;
 }
 
+
 /*
 ** Set the maximum page count for a database if mxPage is positive.
 ** No changes are made if mxPage is 0 or negative.
@@ -2496,7 +2506,6 @@ int sqlite3BtreeSecureDelete(Btree *p, int newFlag){
   sqlite3BtreeLeave(p);
   return b;
 }
-#endif /* !defined(SQLITE_OMIT_PAGER_PRAGMAS) || !defined(SQLITE_OMIT_VACUUM) */
 
 /*
 ** Change the 'auto-vacuum' property of the database. If the 'autoVacuum'