DRDS 支持高效的数据扫描方式,并支持在全表扫描时使用聚合函数进行统计汇总。
常见的扫描场景如下:
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没有分库分表: DRDS 会把原 SQL 传递到后端 MySQL 执行。这种情况下 DRDS 支持任何聚合函数。
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非全表扫描:SQL 经过 DRDS 路由后,发送到单个 MySQL 库上执行。比如说拆分键在 WHERE 中是等于关系时,就会出现非全表扫描。此时同样可以支持任何聚合函数。
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全表扫描:目前支持的聚合函数有 COUNT、MAX、MIN、SUM。另外在全表扫描时同样支持 LIKE、ORDER BY 、LIMIT 以及 GROUP BY 语法。
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并行的全表扫描:如果需要从所有库导出数据,可以通过 SHOW 指令查看表拓扑结构,针对分表并行处理。详见下文。
执行 SHOW TOPOLOGY FROM TABLE_NAME 指令获取表拓扑结构。
mysql:> SHOW TOPOLOGY FROM DRDS_USERS; +------+-------------------+--------------+ | ID | GROUP_NAME | TABLE_NAME | +------+-------------------+--------------+ | 0 | DRDS_00_RDS | drds_users | | 1 | DRDS_01_RDS | drds_users | +------+-------------------+--------------+ 2 rows in set (0.06 sec)
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非分库分表的表默认存储在第0个分库。
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针对 TOPOLOGY 进行单表遍历。
- 第0个分库运行当前 SQL
/!TDDL:node='DRDS_00_RDS'*/ SELECT * FROM DRDS_USERS;
- 第1个分库运行当前 SQL
/!TDDL:node='DRDS_01_RDS'*/ SELECT * FROM DRDS_USERS;
注意:推荐每次扫描前执行 SHOW TOPOLOGY FROM TABLE_NAME 获取最新的表拓扑结构。
若为分表,则 SQL 中的表名为分表名。
比如:
DRDS>SHOW TOPOLOGY FROM c_schedule +--------------+-----------------------------------------------+----------------------+ | ID | GROUP_NAME | TABLE_NAME | +--------------+-----------------------------------------------+----------------------+ | 0 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0000 | c_schedule | +--------------+-----------------------------------------------+----------------------+ 返回行数: 1 耗时: 6.00 ms. DRDS>SHOW TOPOLOGY FROM cs_fb +--------------+-----------------------------------------------+----------------------+ | ID | GROUP_NAME | TABLE_NAME | +--------------+-----------------------------------------------+----------------------+ | 0 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0000 | cs_fb_ojR3_00 | | 1 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0000 | cs_fb_ojR3_01 | | 2 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0000 | cs_fb_ojR3_02 | | 3 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0001 | cs_fb_ojR3_03 | | 4 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0001 | cs_fb_ojR3_04 | | 5 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0001 | cs_fb_ojR3_05 | | 6 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0002 | cs_fb_ojR3_06 | | 7 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0002 | cs_fb_ojR3_07 | | 8 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0002 | cs_fb_ojR3_08 | | 9 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0003 | cs_fb_ojR3_09 | | 10 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0003 | cs_fb_ojR3_10 | | 11 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0003 | cs_fb_ojR3_11 | | 12 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0004 | cs_fb_ojR3_12 | | 13 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0004 | cs_fb_ojR3_13 | | 14 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0004 | cs_fb_ojR3_14 | | 15 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0005 | cs_fb_ojR3_15 | | 16 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0005 | cs_fb_ojR3_16 | | 17 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0005 | cs_fb_ojR3_17 | | 18 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0006 | cs_fb_ojR3_18 | | 19 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0006 | cs_fb_ojR3_19 | | 20 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0006 | cs_fb_ojR3_20 | | 21 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0007 | cs_fb_ojR3_21 | | 22 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0007 | cs_fb_ojR3_22 | | 23 | ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0007 | cs_fb_ojR3_23 | +--------------+-----------------------------------------------+----------------------+ 返回行数: 24 耗时: 18.00 ms.
查询:
/!TDDL:node='ONE_www_ZyiZ_1586333425057BUHJ_VA8S_0006'*/ SELECT * FROM cs_fb_ojR3_19 ;
计算表的方法:比如:
select (2020*12+3)%8
2020为年,年*12,加上月份3,再除以物理库8个。