如何在t-sql上的union子句上获得更好的性能

Question

我有三张桌子。每个表包含超过3M行。我运行以下代码：

SELECT * FROM 
(
    SELECT col_1, col_2, col_3, [date], 1 as type FROM table_1
    UNION
    SELECT col_1, col_2, col_3, [date], 2 as type FROM table_2 
    UNION
    SELECT col_1, col_2, col_3, [date], 3 as type FROM table_3
) AS tb 
tb.[date] BETWEEN (start_date) AND (end_date)  
ORDER BY [date] DESC OFFSET n ROWS FETCH NEXT m ROWS ONLY

但是当我得到大的日期间隔时，查询运行得更慢。例如：当我得到2019-01-01和2019-04-01间隔时，查询运行大约13-14秒：

这个结果非常糟糕。我想在1秒内得到结果。我能做什么？

Answer 1

首先使用UNION ALL而不是UNION：

SELECT *
FROM (SELECT col_1, col_2, col_3, [date], 1 as type FROM table_1
      UNION ALL
      SELECT col_1, col_2, col_3, [date], 2 as type FROM table_2 
      UNION ALL
      SELECT col_1, col_2, col_3, [date], 3 as type FROM table_3
     ) AS tb 
WHERE tb.[date] BETWEEN (start_date) AND (end_date)  
ORDER BY [date] DESC
OFFSET n ROWS FETCH NEXT m ROWS ONLY;

SQL会导致使用UNION删除重复项的开销。 UNION ALL不会产生这种开销。

此外，每个表格中的date指数应该有所帮助。 SQL Server有一个很好的优化器，通常会将这些条件推送到UNION / UNION ALL子查询中的各个查询。

Answer 2

我建议在每个表上创建一个覆盖索引，类似于：

CREATE INDEX ix1 ON table_1 (date) INCLUDE (column1, column2, column3)

这应该有助于WHERE子句。另外，SQL Server不必触及表格，因为索引中存在所有必需的信息。

---

这是对此的又一次尝试。假设OFFSET n ROWS FETCH NEXT m ROWS ONLY在开始日期和结束日期之间匹配很小比例的行，请编写如下查询：

WITH cte1 AS (
    -- find the first date after n + m window
    SELECT date
    FROM (
        SELECT date FROM table_1 UNION ALL
        SELECT date FROM table_2 UNION ALL
        SELECT date FROM table_3
    ) AS x
    WHERE date BETWEEN '2019-01-01' AND '2019-04-01'
    ORDER BY date DESC OFFSET (n + m) ROWS FETCH NEXT 1 ROW ONLY
), cte2 AS (
    SELECT date, column_1, column_2, column_3, 1 AS type FROM table_1 UNION ALL
    SELECT date, column_1, column_2, column_3, 1 AS type FROM table_2 UNION ALL
    SELECT date, column_1, column_2, column_3, 1 AS type FROM table_3
)
SELECT *
FROM cte2
WHERE date <= '2019-04-01' AND date > (SELECT date FROM cte1)
ORDER BY date DESC OFFSET n ROWS FETCH NEXT m ROWS ONLY

Answer 3

我不确定查询规划器是否足够聪明，可以通过union之外的where子句来限制union的结果，所以尝试将日期条件移动到union中的每个查询中，这样你就不会联合在运行条件之前，三个表的整体在一起：

SELECT * FROM 
(
    SELECT col_1, col_2, col_3, [date], 1 as type FROM table_1 where table_1.[date] between (start_date) and (end_date)
    UNION
    SELECT col_1, col_2, col_3, [date], 2 as type FROM table_2 where table_2.[date] between (start_date) and (end_date) 
    UNION
    SELECT col_1, col_2, col_3, [date], 3 as type FROM table_3 where table_3.[date] between (start_date) and (end_date)
) AS tb 
ORDER BY [date] DESC OFFSET n ROWS FETCH NEXT m ROWS ONLY