在大数据系统中,如无特别需要,建议 collation 指定为 utf8mb4_bin。
utf8mb4_bin是一种二进制的排序规则,比较字符串时直接比较字符串的二进制值,不需要进行复杂的字符比较和排序运算,这样可以有效减少CPU的使用,提高查询效率,特别是在涉及到大量数据操作时,性能优势更为明显。
在 AP 场景使用 OceanBase MySQL 租户模式时你可以在租户级别设置这两个值作为默认配置,以获得最佳的默认性能:
sql
set global collation_connection = utf8mb4_bin;
set global collation_server = utf8mb4_bin;
utf8mb4 是编码格式,设置 collation 为 bin 不会影响内存的存储格式,不会造成字符集不兼容等问题,它仅仅影响排序过程中的排序规则。
使用 bin 排序和不使用 bin 排序的详细对比如下,可以看到,使用 bin 排序,性能可以提升 15%。按照我的经验,在实际应用场景中,性能提升比例可能更大!
sql
OceanBase(admin@test)>show variables like 'collation_server';
+------------------+--------------------+
| Variable_name | Value |
+------------------+--------------------+
| collation_server | utf8mb4_general_ci |
+------------------+--------------------+
1 row in set (0.007 sec)
OceanBase(admin@test)>create table t1 (c1 bigint primary key, c2 varchar(10));
Query OK, 0 rows affected (0.102 sec)
OceanBase(admin@test)>create table t2 (c1 bigint primary key, c2 varchar(10)) charset=utf8mb4 collate=utf8mb4_bin;
Query OK, 0 rows affected (0.089 sec)
OceanBase(admin@test)>show create table t1\G
*************************** 1. row ***************************
Table: t1
Create Table: CREATE TABLE `t1` (
`c1` bigint(20) NOT NULL,
`c2` varchar(10) DEFAULT NULL,
PRIMARY KEY (`c1`)
) DEFAULT CHARSET = utf8mb4 ROW_FORMAT = DYNAMIC COMPRESSION = 'zstd_1.3.8' REPLICA_NUM = 2 BLOCK_SIZE = 16384 USE_BLOOM_FILTER = FALSE TABLET_SIZE = 134217728 PCTFREE = 0
1 row in set (0.011 sec)
OceanBase(admin@test)>show create table t2\G
*************************** 1. row ***************************
Table: t2
Create Table: CREATE TABLE `t2` (
`c1` bigint(20) NOT NULL,
`c2` varchar(10) COLLATE utf8mb4_bin DEFAULT NULL,
PRIMARY KEY (`c1`)
) DEFAULT CHARSET = utf8mb4 COLLATE = utf8mb4_bin ROW_FORMAT = DYNAMIC COMPRESSION = 'zstd_1.3.8' REPLICA_NUM = 2 BLOCK_SIZE = 16384 USE_BLOOM_FILTER = FALSE TABLET_SIZE = 134217728 PCTFREE = 0
1 row in set (0.011 sec)
OceanBase(admin@test)>insert /*+ append enable_parallel_dml parallel(4) */ into t1 select random(), randstr(1, 6) from table(generator(1000000));
Query OK, 1000000 rows affected (8.629 sec)
Records: 1000000 Duplicates: 0 Warnings: 0
OceanBase(admin@test)>insert /*+ append enable_parallel_dml parallel(4) */ into t2 select * from t1;
Query OK, 1000000 rows affected (7.115 sec)
Records: 1000000 Duplicates: 0 Warnings: 0
OceanBase(admin@test)>select * from t1 order by c2 limit 999998, 1;
+----------------------+------+
| c1 | c2 |
+----------------------+------+
| -8614823888367694260 | 5 |
+----------------------+------+
1 row in set (1.050 sec)
OceanBase(admin@test)>select * from t2 order by c2 limit 999998, 1;
+----------------------+------+
| c1 | c2 |
+----------------------+------+
| -8614823888367694260 | 5 |
+----------------------+------+
1 row in set (0.876 sec)
OceanBase(admin@test)>select * from t1 order by c2 limit 999998, 1;
+----------------------+------+
| c1 | c2 |
+----------------------+------+
| -8614823888367694260 | 5 |
+----------------------+------+
1 row in set (1.053 sec)
OceanBase(admin@test)>select * from t2 order by c2 limit 999998, 1;
+----------------------+------+
| c1 | c2 |
+----------------------+------+
| -8614823888367694260 | 5 |
+----------------------+------+
1 row in set (0.844 sec)
通过上面的对比可以看到,由于 t2 表使用的 collation 为 utf8mb4_bin,其排序耗时只需要0.844秒,使用了 utf8mb4_general_c1 的 t1 表排序耗时则需要 1.053 秒,差距明显。
在 Oracle 模式下,我们通过设置变量 NLS_SORT 为 binary 也可以达到相同效果。默认情况下,NLS_SORT 已经被设置为 binary。