Use a 4-byte constant for the presence index key. Set the index db hash

function to simply pass thru the key value, which is already a hash value.

servers/slapd/back-bdb/dbcache.c

@@ -17,6 +17,23 @@
 
 #include "slap.h"
 #include "back-bdb.h"
+#include "lutil_hash.h"
+
+/* Pass-thru hash function. Since the indexer is already giving us hash
+ * values as keys, we don't need BDB to re-hash them.
+ */
+#if LUTIL_HASH_BYTES == 4
+static u_int32_t
+bdb_db_hash(
+	DB *db,
+	const void *bytes,
+	u_int32_t length
+)
+{
+	u_int32_t *ret = (u_int32_t *)bytes;
+	return *ret;
+}
+#endif
 
 int
 bdb_db_cache(
@@ -69,6 +86,9 @@ bdb_db_cache(
 	}
 
 	rc = db->bdi_db->set_pagesize( db->bdi_db, BDB_PAGESIZE );
+#if LUTIL_HASH_BYTES == 4
+	rc = db->bdi_db->set_h_hash( db->bdi_db, bdb_db_hash );
+#endif
 #ifdef BDB_IDL_MULTI
 	rc = db->bdi_db->set_flags( db->bdi_db, DB_DUP | DB_DUPSORT );
 #endif

servers/slapd/back-bdb/index.c

@@ -14,6 +14,10 @@
 
 #include "slap.h"
 #include "back-bdb.h"
+#include "lutil_hash.h"
+
+static char presence_keyval[LUTIL_HASH_BYTES] = {0,0,0,1};
+struct berval presence_key = {LUTIL_HASH_BYTES, presence_keyval};
 
 static slap_mask_t index_mask(
 	Backend *be,
@@ -169,7 +173,7 @@ static int indexer(
 	if( rc != LDAP_SUCCESS ) return rc;
 
 	if( IS_SLAP_INDEX( mask, SLAP_INDEX_PRESENT ) ) {
-		rc = bdb_key_change( be, db, txn, atname, id, op );
+		rc = bdb_key_change( be, db, txn, &presence_key, id, op );
 		if( rc ) {
 			goto done;
 		}