合并多个选择而没有并集或全部并集

Question

我有一个庞大的查询，该查询曾经对很多表（每个表有成千上万行）执行UNION ALL，然后在返回之前输出到临时表。

旧表格：

 SELECT *
FROM   (SELECT `a` AS `Human readable A`,
               `b` AS `Human readable B`,
               `c` AS `Human readable C`,
        FROM   `table1`
        UNION ALL
        SELECT
               `a` AS `Human readable A`,
               `b` AS `Human readable B`,
               `c` AS `Human readable C`,
        FROM   `table2`
        UNION ALL
        SELECT
              `a` AS `Human readable A`,
              `b` AS `Human readable B`,
              `c` AS `Human readable C`,

        FROM `table3`
) AS temp_table 

该查询几乎杀死了数据库（查询需要20分钟到61分钟之间的任何时间），在这段时间内CPU完全耗尽。

我发现为每个表运行一个单独的SELECT语句最多只花了几秒钟 ，并决定将它们合并到位于不同物理服务器上的应用程序级别，这是一个额外的好处（下面的伪代码）。

    $result1 =  SELECT
                      `a` AS `Human readable A`,
                      `b` AS `Human readable B`,
                      `c` AS `Human readable C`,

                FROM `table1`

    $result2 =  SELECT
                      `a` AS `Human readable A`,
                      `b` AS `Human readable B`,
                      `c` AS `Human readable C`,

                FROM `table2`

    $result3 =  SELECT
                      `a` AS `Human readable A`,
                      `b` AS `Human readable B`,
                      `c` AS `Human readable C`,

                FROM `table3`

$result4 = merge($result1, $result2, $result3)

但是，我觉得这有点不安全，因为查询可能会更新这些单独的选择查询之间的数据。 有没有一种方法可以改善我的选择语句查询集，使其仅作为一个事务处理 （无需写操作），因此所有数据将被共享的读取锁锁定并返回。

附加信息

我预计原始表单会花费更长的时间，因为它花费大量的CPU时间来重新创建/排序组合表中的索引，而我不需要这样做（我只需要将结果附加在一起）。

• 所有表的结构完全相同
• 请注意，每个表大约有34 a AS Human readable A a AS。由于数据与不同的项目相关，因此数据被分为不同的表。
• 此特定查询中有20个联合（21个表）。
• 使用InnoDB表获取数据。 我知道这在CPU上比MyIsam更为密集，但是在了解了MyIsam的各种缺点之后，我不愿意切换存储引擎。
• 没有WHERE子句（通过拆分成表，数据已经“预先分组”）

Answer 1

考虑到您的约束，最好的调用是在发出每个连续的SELECT之前显式地锁定表：

SET autocommit=0; -- optional, but this is where and how you must start the transaction if you need one
LOCK TABLES t1 READ, t2 READ, t3 READ;
SELECT a FROM t1;
SELECT a FROM t2;
SELECT a FROM t3;
UNLOCK TABLES; -- beware: implicit COMMIT

除非有某种法律要求将这些数据保存在多个表中，否则您确实应该坚持验证所有这些表合并到一个表中。

Answer 2

我以为我会通过代码示例提供两种可能的解决方案及其各种好处。 解决方案之一是从RandomSeed的答案中“窃取”：

if ($READING_ONLY_INNODB_TABLES)
{
    /**
     * - Since these tables are innodb, we can make use of its 'REPEATABLE READ'
     * isolation level
     * - Locking the entire tables directly is slightly faster, but this method 
     * allows us to have a consistent view of the database without implementing
     * ANY locks (which would block other processes). 
     * It may be easier to think of them as locking as this results in the same 
     * result in terms of consistency (innodb even handles phantom reads at this level)
     * - MyIsam does not support REPEATABLE READ, hence this cannot be used for it
     */
   $query = 
       'START TRANSACTION WITH CONSISTENT SNAPSHOT;'. # --auto sets "SET autocommit=0"
       $queries_string . # --This is the series of selects
       'COMMIT;';
}
else
{
    /**
     * This is a lower resource intensive, 'generic' way (works with MyISAM) that will wait until it can read lock
     * all the tables before reading. This way we should 'force' a 
     * 'repeatable read'/consitent view.
     */
    $query = 
        'SET autocommit=0;'. # starts the transaction
        'LOCK TABLES ' . $lock_tables_string . ';' . # Automatically commits anything before this
        $queries_string . # This is the series of selects from the tables we just locked
        'UNLOCK TABLES;'; # commits a transaction if any tables currently have been locked with LOCK TABLES    
}