how to make selecting random rows in oracle faster with table with millions of rows

Question

Is there a way to make selecting random rows faster in oracle with a table that has million of rows. I tried to use sample(x) and dbms_random.value and its taking a long time to run.

Thanks!

Answer 1

Using appropriate values of sample(x) is the fastest way you can. It's block-random and row-random within blocks, so if you only want one random row:

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

I'm using a subpartitioned table, and I'm getting pretty good randomness even grabbing multiple rows:

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

I suspect you should probably tune your SAMPLE clause to use an appropriate sample size for what you're fetching.