memtx: create tree with block cardinalities if fast_offset is set

This patch makes the memtx tree index create different trees depending
on the `fast_offset` index option. This is the first step towards the
logarithmic select and iteration with offset and count implementation.

NO_DOC=see the next commit
NO_CHANGELOG=see the next commit

src/box/lua/space.cc

@@ -566,8 +566,13 @@ lbox_fillspace(struct lua_State *L, struct space *space, int i)
 			lua_setfield(L, -2, "hint");
 		}
 
-		lua_pushnil(L);
-		lua_setfield(L, -2, "fast_offset");
+		if (space_is_memtx(space) && index_def->type == TREE) {
+			lua_pushboolean(L, index_opts->fast_offset);
+			lua_setfield(L, -2, "fast_offset");
+		} else {
+			lua_pushnil(L);
+			lua_setfield(L, -2, "fast_offset");
+		}
 
 		if (index_opts->func_id > 0) {
 			lua_pushstring(L, "func");

src/box/memtx_engine.cc

@@ -2014,6 +2014,8 @@ memtx_index_def_change_requires_rebuild(struct index *index,
 		return true;
 	if (old_def->opts.hint != new_def->opts.hint)
 		return true;
+	if (old_def->opts.fast_offset != new_def->opts.fast_offset)
+		return true;
 
 	const struct key_def *old_cmp_def, *new_cmp_def;
 	if (index_depends_on_pk(index)) {

src/box/memtx_space.c

@@ -761,11 +761,6 @@ memtx_space_check_index_def(struct space *space, struct index_def *index_def)
 {
 	struct key_def *key_def = index_def->key_def;
 
-	if (index_def->opts.fast_offset) {
-		diag_set(ClientError, ER_UNSUPPORTED, "memtx",
-			 "logarithmic select with offset");
-		return -1;
-	}
 	if (key_def->is_nullable) {
 		if (index_def->iid == 0) {
 			diag_set(ClientError, ER_NULLABLE_PRIMARY,
@@ -787,6 +782,13 @@ memtx_space_check_index_def(struct space *space, struct index_def *index_def)
 				 "HASH index must be unique");
 			return -1;
 		}
+		if (index_def->opts.fast_offset) {
+			diag_set(ClientError, ER_MODIFY_INDEX,
+				 index_def->name, space_name(space),
+				 "HASH index does not support "
+				 "logarithmic select with offset");
+			return -1;
+		}
 		if (key_def->is_multikey) {
 			diag_set(ClientError, ER_MODIFY_INDEX,
 				 index_def->name, space_name(space),
@@ -816,6 +818,13 @@ memtx_space_check_index_def(struct space *space, struct index_def *index_def)
 				 "RTREE index can not be unique");
 			return -1;
 		}
+		if (index_def->opts.fast_offset) {
+			diag_set(ClientError, ER_MODIFY_INDEX,
+				 index_def->name, space_name(space),
+				 "RTREE index does not support "
+				 "logarithmic select with offset");
+			return -1;
+		}
 		if (key_def->parts[0].type != FIELD_TYPE_ARRAY) {
 			diag_set(ClientError, ER_MODIFY_INDEX,
 				 index_def->name, space_name(space),
@@ -849,6 +858,13 @@ memtx_space_check_index_def(struct space *space, struct index_def *index_def)
 				 "BITSET can not be unique");
 			return -1;
 		}
+		if (index_def->opts.fast_offset) {
+			diag_set(ClientError, ER_MODIFY_INDEX,
+				 index_def->name, space_name(space),
+				 "BITSET index does not support "
+				 "logarithmic select with offset");
+			return -1;
+		}
 		if (key_def->parts[0].type != FIELD_TYPE_UNSIGNED &&
 		    key_def->parts[0].type != FIELD_TYPE_STRING &&
 		    key_def->parts[0].type != FIELD_TYPE_VARBINARY) {