MyRockscollation限制

zysql發表於2018-09-21

title: MySQL · myrocks · collation 限制

author: 張遠

背景

MyRocks中的資料是按索引列以memcmp方式進行排序的。對於一些數字型別，需要進行轉化才能直接通過memcmp進行比較， 例如有符號數在計算機中是用補碼錶示的，那麼如果負數和正數直接按位元組比較，結果負數會比正數大，實際儲存時會將符號會反轉儲存，讀取時再轉化回來。對於字元型別，處理更加複雜，涉及到字符集的轉換。 記錄格式可以參考[[1]](https://github.com/facebook/mysql-5.6/wiki/MyRocks-record-format), [[2]](https://yq.aliyun.com/articles/62648)

MyRocks索引欄位如果包含字元型別，預設只支援binary collation，binary、latin1_bin、 utf8_bin其中的一種

# Error
create table t1(c1 int primary key, c2 varchar(10) unique) engine =rocksdb character set gbk;
ERROR 3046 (HY000): Unsupported collation on string indexed column test.t1.c2 Use binary collation (binary, latin1_bin, utf8_bin).

# Error
create table t1(c1 int primary key, c2 varchar(10) unique) engine =rocksdb character set utf8 collate utf8_general_ci;
ERROR 3046 (HY000): Unsupported collation on string indexed column test.t1.c2 Use binary collation (binary, latin1_bin, utf8_bin).

# OK
create table t1(c1 int primary key, c2 varchar(10) unique) engine =rocksdb character set utf8 collate utf8_bin;
Query OK, 0 rows affected (0.00 sec)

通過設定rocksdb_strict_collation_check引數為OFF可以跳出binary collation的限制

set global rocksdb_strict_collation_check=OFF;

# OK
create table t1(c1 int primary key, c2 varchar(10) unique) engine =rocksdb character set gbk;
Query OK, 0 rows affected (0.01 sec)

問題

MyRocks和InnoDB一樣支援covering index. MyRocks在使用二級索引查詢的時候，應儘量使用covering index, 因為MyRocks回表通過主鍵隨機查詢資料的開銷比較大。

例如以下場景，idx1作為convering index被使用


create table t1(c1 int primary key, c2 varchar(10) unique) engine =rocksdb character set utf8 collate utf8_bin;

insert into t1 values(1,`ab`);
insert into t1 values(2,`cd`);

# covering index
explain select c2 from t1 where c2=`ab`;
+----+-------------+-------+-------+---------------+------+---------+-------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref   | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+-------+------+-------------+
|  1 | SIMPLE      | t1    | const | c2            | c2   | 33      | const |    1 | Using index |
+----+-------------+-------+-------+---------------+------+---------+-------+------+-------------+

然而設定set global rocksdb_strict_collation_check=OFF;在某些情況下會導致我們無法使用covering index.

set global rocksdb_strict_collation_check=ON;

#Error
create table t1(c1 int primary key, c2 int, c3 varchar(10), key idx1(c2,c3)) engine =rocksdb  character set utf8 collate utf8_general_ci;
ERROR 3046 (HY000): Unsupported collation on string indexed column test.t1.c3 Use binary collation (binary, latin1_bin, utf8_bin).

set global rocksdb_strict_collation_check=OFF;

# OK
create table t1(c1 int primary key, c2 varchar(10) unique) engine =rocksdb character set utf8 collate utf8_general_ci;
Query OK, 0 rows affected (0.00 sec)

insert into t1 values(1,1,`ab`);
insert into t1 values(2,2,`cd`);


insert into t1 values(1,`ab`);
insert into t1 values(2,`cd`);

# non-covering index
explain select c2 from t1 where c2=`ab`;
+----+-------------+-------+-------+---------------+------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+------+---------+-------+------+-------+
|  1 | SIMPLE      | t1    | const | c2            | c2   | 33      | const |    1 | NULL  |
+----+-------------+-------+-------+---------------+------+---------+-------+------+-------+
1 row in set (0.00 sec)

原因

MyRocks二級索引由於collation的關係導致查詢沒有走covering index. MyRocks中索引列需要轉化為memcomparable的形式，轉化分為以下三種情況

  • 1) 直接轉換，不需要儲存額外資訊

    例如  binary、latin1_bin、 utf8_bin三種collation可以直接轉換

這種情況二級索引列資料可以完整的從二級索引中取到，不影響covering index使用

  • 2) 間接轉換，需在value中增加unpack_info

    例如latin1_general_ci,latin2_general_ci, ascii_general_ci,greek_general_ci等collation，具體可以參考函式rdb_is_collation_supported

這種情況二級索引列資料可以從key和unpack_info中解析取到，也不影響covering index使用

create table t1(c1 int primary key, c2 varchar(10) unique) engine =rocksdb character set latin1 collate latin1_general_ci;

insert into t1 values(1,`ab`);
insert into t1 values(2,`cd`);

# covering index
explain select c2 from t1 where c2=`ab`;
+----+-------------+-------+-------+---------------+------+---------+-------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref   | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+-------+------+-------------+
|  1 | SIMPLE      | t1    | const | c2            | c2   | 13      | const |    1 | Using index |
+----+-------------+-------+-------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)

  • 3) 無法轉換

    除1，2兩種情況外的collation, 例如utf8_general_ci

此時從二級索引中獲取不到key的完整資訊，需要從主鍵索引上獲取，因此不能走covering index

create table t1(c1 int primary key, c2 varchar(10) unique) engine =rocksdb character set utf8 collate utf8_general_ci;

 insert into t1 values(1,`ab`);
 insert into t1 values(2,`cd`);

# non-covering index
explain select c2 from t1 where c2=`ab`;
+----+-------------+-------+-------+---------------+------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+------+---------+-------+------+-------+
|  1 | SIMPLE      | t1    | const | c2            | c2   | 33      | const |    1 | NULL  |
+----+-------------+-------+-------+---------------+------+---------+-------+------+-------+
1 row in set (0.00 sec)

此時的資料獲取路徑如下

0  myrocks::ha_rocksdb::convert_record_from_storage_format 
1  myrocks::ha_rocksdb::get_row_by_rowid
2  myrocks::ha_rocksdb::get_row_by_rowid
3  myrocks::ha_rocksdb::read_row_from_secondary_key 
4  myrocks::ha_rocksdb::index_read_map_impl
5  myrocks::ha_rocksdb::index_read_map
6  handler::index_read_idx_map
7  handler::ha_index_read_idx_map
8  join_read_const
9  join_read_const_table
10 make_join_statistics
11 JOIN::optimize
12 mysql_execute_select
13 mysql_select
14 handle_select
15 execute_sqlcom_select
16 mysql_execute_command

轉換的具體實現可以參考函式Rdb_field_packing::setup

MyRocks從索引讀取資料時，不能僅通過keyread_only來判斷是否可以使用covering index, 還需要判斷是否存在collation列資料轉換的問題，如果訪問的列無法轉換就不能使用covering index.

MyRocks會在value中儲存covered_bitmap,表示索引列是否可以轉換, read_row_from_secondary_key/secondary_index_read讀取時會根據covered_bitmap來決定是否能使用covering index

bool Rdb_key_def::covers_lookup(TABLE *const table,
                                const rocksdb::Slice *const unpack_info,
                                const MY_BITMAP *const lookup_bitmap) const {
  DBUG_ASSERT(lookup_bitmap != nullptr);
  if (!use_covered_bitmap_format() || lookup_bitmap->bitmap == nullptr) {
    return false;
  }

  Rdb_string_reader unp_reader = Rdb_string_reader::read_or_empty(unpack_info);

  // Check if this unpack_info has a covered_bitmap
  const char *unpack_header = unp_reader.get_current_ptr();
  const bool has_covered_unpack_info =
      unp_reader.remaining_bytes() &&
      unpack_header[0] == RDB_UNPACK_COVERED_DATA_TAG;
  if (!has_covered_unpack_info ||
      !unp_reader.read(RDB_UNPACK_COVERED_HEADER_SIZE)) {
    return false;
  }

  MY_BITMAP covered_bitmap;
  my_bitmap_map covered_bits;
  bitmap_init(&covered_bitmap, &covered_bits, MAX_REF_PARTS, false);
  covered_bits = rdb_netbuf_to_uint16((const uchar *)unpack_header +
                                      sizeof(RDB_UNPACK_COVERED_DATA_TAG) +
                                      RDB_UNPACK_COVERED_DATA_LEN_SIZE);

  return bitmap_is_subset(lookup_bitmap, &covered_bitmap);
}

總結

MyRocks在字元collation上的限制需要引起我們關注，使用不當會影響查詢效率。


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