我有以下连接查询,在my_table_name中记录数为500,大约需要50到60秒。
您是否可以建议更好的方法来编写此查询以提高性能。
SELECT DISTINCT REGEXP_SUBSTR(STD_DEP,'[^,]+', 1, LEVEL) AS DEP_STD_ID
FROM my_table_name
WHERE std_id = 242
CONNECT BY REGEXP_SUBSTR(STD_DEP,'[^,]+', 1, LEVEL) IS NOT NULL;
在上面的查询中:STD_DEP是一个逗号分隔的字段,它将包含所有依赖的std_id。
答案 0 :(得分:0)
在Oracle中有many, many ways分隔分隔字符串。
不使用正则表达式的是:
WITH bounds ( id, list, start_pos, end_pos, lvl ) AS (
SELECT id, list, 1, INSTR( list, ',' ), 1
FROM my_table_name
WHERE std_id = 242
UNION ALL
SELECT id,
list,
end_pos + 1,
INSTR( list, ',', end_pos + 1 ),
lvl + 1
FROM bounds
WHERE end_pos > 0
)
SELECT id,
SUBSTR(
list,
start_pos,
DECODE( end_pos, 0, LENGTH( list ) + 1, end_pos ) - start_pos
) AS item,
lvl
FROM bounds
ORDER BY id, lvl;