如何使用具有数百万行的表更快地在oracle中选择随机行

Question

有没有办法在oracle中使用具有数百万行的表更快地选择随机行.我尝试使用sample(x)和dbms_random.value,并且需要很长时间才能运行.

谢谢!

Answer 1

使用适当的值sample(x)是最快的方法.它是块内的块随机和行随机,所以如果你只想要一个随机行:

select dbms_rowid.rowid_relative_fno(rowid) as fileno,
       dbms_rowid.rowid_block_number(rowid) as blockno,
       dbms_rowid.rowid_row_number(rowid) as offset
  from (select rowid from [my_big_table] sample (.01))
 where rownum = 1

我正在使用一个子分区表,即使抓住多行,我也会得到很好的随机性:

select dbms_rowid.rowid_relative_fno(rowid) as fileno,
       dbms_rowid.rowid_block_number(rowid) as blockno,
       dbms_rowid.rowid_row_number(rowid) as offset
  from (select rowid from [my_big_table] sample (.01))
 where rownum <= 5

    FILENO    BLOCKNO     OFFSET
---------- ---------- ----------
       152    2454936         11
       152    2463140         32
       152    2335208          2
       152    2429207         23
       152    2746125         28

我怀疑你应该调整你的SAMPLE条款,使用适当的样本大小来获取你的内容.