Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

CHAPTER 14 ■ PARALLEL EXECUTION 621
■Note The ROWS, BYTES, and TIME columns were removed from this plan output to allow it to fit on the
page. The aggregate time for the query, however, was 00:00:54 as opposed to the previous estimate of
00:06:29 for the serial plan. Remember, these are estimates, not promises! Also, this is the plan output from
Oracle 10g—the plan output from Oracle9i would have less detail (four steps instead of seven), but the net
effect is identical.
If you read this plan from the bottom up, starting at ID=6, it shows the steps described in
Figure 14-1. The full table scan would be split up into many “smaller scans” (step 5). Each of
those would aggregate their COUNT(STATUS) values (step 4). These subresults would be transmitted
to the parallel query coordinator (steps 2 and 3), which would aggregate these results
further (step 1) and output the answer.
If you were to actually execute this query on a freshly started system (where no other parallel
execution had been done), you might observe the following. Here we are using the Linux
ps command to find processes that represent parallel query processes (we expect to find
none), running the query with parallel execution enabled, and then looking for those
processes again:
big_table@ORA10GR1> host ps -auxww | grep '^ora10gr1.*ora_p00._ora10g'
big_table@ORA10GR1> select count(status) from big_table;
COUNT(STATUS)
-------------
10000000
big_table@ORA10GR1> host ps -auxww | grep '^ora10gr1.*ora_p00._ora10g'
ora10gr1 3411 35.5 0.5 1129068 12200 ? S 13:27 0:02 ora_p000_ora10gr1
ora10gr1 3413 28.0 0.5 1129064 12196 ? S 13:27 0:01 ora_p001_ora10gr1
ora10gr1 3415 26.0 0.5 1129064 12196 ? S 13:27 0:01 ora_p002_ora10gr1
ora10gr1 3417 23.3 0.5 1129044 12212 ? S 13:27 0:01 ora_p003_ora10gr1
ora10gr1 3419 19.5 0.5 1129040 12228 ? S 13:27 0:01 ora_p004_ora10gr1
ora10gr1 3421 19.1 0.5 1129056 12188 ? S 13:27 0:01 ora_p005_ora10gr1
ora10gr1 3423 19.0 0.5 1129056 12164 ? S 13:27 0:01 ora_p006_ora10gr1
ora10gr1 3425 21.6 0.5 1129048 12204 ? S 13:27 0:01 ora_p007_ora10gr1
As we can see, there are now eight parallel execution servers that have been started by
Oracle. If we are curious enough to want to “watch” parallel query, we can easily do so using
two sessions. In the session we will run the parallel query in, we’ll start by determining
our SID:
big_table@ORA10GR1> select sid from v$mystat where rownum = 1;
SID
----------
162