Home > Oracle > how do i limit the number of rows returned by an oracle query after ordering

how do i limit the number of rows returned by an oracle query after ordering

Source http://stackoverflow.com/questions/470542/how-do-i-limit-the-number-of-rows-returned-by-an-oracle-query-after-ordering

I did some performance testing between:

A.) Asktom

select * from (
  select a.*, ROWNUM rnum from (
    <select statemenet with order by clause>
  ) a where rownum <= MAX_ROW
) where rnum >= MIN_ROW

B.) Analytic approach

select * from (
  <select statemenet with order by clause>
) where myrow between MIN_ROW and MAX_ROW

C.) Short alternative

select * from (
  select statement, rownum as RN with order by clause
) where a.rn >= MIN_ROW and a.rn <= MAX_ROW

Results:

Table had 10 million records, sort was on an unindexed datetime row:

  • Explain plan showed same value for all three selects (323168)
  • But the winner is AskTom (with analytic following close behind)

Selecting first 10 rows took:

  • AskTom: 28-30 seconds
  • Analytical: 33-37 seconds
  • Short alternative: 110-140 seconds

Selecting rows betwwen 100.000 and 100.010:

  • AskTom: 60 seconds
  • Analytical: 100 seconds

Selecting rows between 9.000.000 and 9.000.010:

  • AskTom: 130 seconds
  • Analytical: 150 seconds
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