有没有办法使用具有百万行的表在oracle中更快地选择随机行.我试图使用sample(x)和dbms_random.value,它花费很长时间才能运行.
谢谢!
使用适当的sample(x)值是您可以最快的方式.它在块内是块随机的和行随机的,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,即使抓取多行,我也获得了非常好的随机性:
select dbms_rowid.rowid_relative_fno(rowid) as fileno,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子句,使用适当的样本大小来取得你想要的东西.