如何使用OR條件提高SQL JOIN的性能

Question

復雜的JOIN查詢為：

SELECT 
  FIRST.NAME,
  SECOND.FIRST_NAME,
  SECOND.LAST_NAME
FROM FIRST_TABLE FIRST
LEFT JOIN SECOND_TABLE SECOND 
  ON (SECOND.FIRST_NAME = FIRST.NAME OR SECOND.LAST_NAME = FIRST.NAME)

將導致性能下降。

如何獲得更好的性能？

Answer 1

這回答了問題的原始版本。

exists可能會更好：

SELECT FIRST.ID, FIRST.NAME
FROM FIRST_TABLE FIRST 
WHERE EXISTS (SELECT 1 FROM SECOND_TABLE SECOND WHERE SECOND.FIRST_NAME = FIRST.NAME) OR
      EXISTS (SELECT 1 FROM SECOND_TABLE SECOND WHERE  SECOND.LAST_NAME = FIRST.NAME);

為了提高性能，您需要在SECOND_TABLE(LAST_NAME)和SECOND_TABLE(FIRST_NAME)上SECOND_TABLE(LAST_NAME)索引。

Answer 2

您可以嘗試一下，看看會得到什么。 它在很大程度上取決於FIRST_NAME和LAST_NAME上的索引，但是避免使用OR 。

SELECT 
  FIRST.NAME,
  SECOND.FIRST_NAME,
  SECOND.LAST_NAME
FROM FIRST_TABLE FIRST
  LEFT JOIN (
     SELECT S1.FIRST_NAME as TEST_NAME,
            S1.FIRST_NAME,
            S1.LAST_NAME       
     FROM   SECOND_TABLE S1
     WHERE  S1.FIRST_NAME <> S1.LAST_NAME
     UNION ALL
     SELECT S2.LAST_NAME as TEST_NAME,
            S2.FIRST_NAME,
            S2.LAST_NAME       
     FROM   SECOND_TABLE S2
   ) AS SECOND
  ON FIRST.NAME = SECOND.TEST_NAME