ITS#8221 don't merge branch pages needlessly

libraries/liblmdb/mdb.c

@@ -7887,17 +7887,24 @@ mdb_rebalance(MDB_cursor *mc)
 {
 	MDB_node	*node;
 	int rc;
-	unsigned int ptop, minkeys;
+	unsigned int ptop, minkeys, thresh;
 	MDB_cursor	mn;
 	indx_t oldki;
 
+	if (IS_BRANCH(mc->mc_pg[mc->mc_top])) {
+		minkeys = 1;
+		thresh = 1;
+	} else {
+		minkeys = 2;
+		thresh = FILL_THRESHOLD;
+	}
 	minkeys = 1 + (IS_BRANCH(mc->mc_pg[mc->mc_top]));
 	DPRINTF(("rebalancing %s page %"Z"u (has %u keys, %.1f%% full)",
 	    IS_LEAF(mc->mc_pg[mc->mc_top]) ? "leaf" : "branch",
 	    mdb_dbg_pgno(mc->mc_pg[mc->mc_top]), NUMKEYS(mc->mc_pg[mc->mc_top]),
 		(float)PAGEFILL(mc->mc_txn->mt_env, mc->mc_pg[mc->mc_top]) / 10));
 
-	if (PAGEFILL(mc->mc_txn->mt_env, mc->mc_pg[mc->mc_top]) >= FILL_THRESHOLD &&
+	if (PAGEFILL(mc->mc_txn->mt_env, mc->mc_pg[mc->mc_top]) >= thresh &&
 		NUMKEYS(mc->mc_pg[mc->mc_top]) >= minkeys) {
 		DPRINTF(("no need to rebalance page %"Z"u, above fill threshold",
 		    mdb_dbg_pgno(mc->mc_pg[mc->mc_top])));
@@ -8032,7 +8039,7 @@ mdb_rebalance(MDB_cursor *mc)
 	 * (A branch page must never have less than 2 keys.)
 	 */
 	minkeys = 1 + (IS_BRANCH(mn.mc_pg[mn.mc_top]));
-	if (PAGEFILL(mc->mc_txn->mt_env, mn.mc_pg[mn.mc_top]) >= FILL_THRESHOLD && NUMKEYS(mn.mc_pg[mn.mc_top]) > minkeys) {
+	if (PAGEFILL(mc->mc_txn->mt_env, mn.mc_pg[mn.mc_top]) >= thresh && NUMKEYS(mn.mc_pg[mn.mc_top]) > minkeys) {
 		rc = mdb_node_move(&mn, mc);
 		if (mc->mc_ki[ptop]) {
 			oldki++;