qsort, once again - Mailing list pgsql-hackers

From Tom Lane
Subject qsort, once again
Msg-id 19164.1142537875@sss.pgh.pa.us
Whole thread Raw
Responses Re: qsort, once again
List pgsql-hackers
I was just looking at the behavior of src/port/qsort.c on the test case
that Jerry Sievers was complaining about in pgsql-admin this morning.
I found out what the real weak spot is: it's got nothing directly to do
with good or bad pivots, it's this code right here:
   if (swap_cnt == 0)   {                            /* Switch to insertion sort */       for (pm = (char *) a + es; pm
<(char *) a + n * es; pm += es)           for (pl = pm; pl > (char *) a && cmp(pl - es, pl) > 0;                pl -=
es)              swap(pl, pl - es);       return;   }

In other words, if qsort hits a subfile for which the chosen pivot is a
perfect pivot (no swaps are necessary), it switches to insertion sort.
Which is O(N^2).  In Jerry's test case this happens on a subfile of
736357 elements, and you can say goodnight to that process ....

What I'm thinking is that we ought to have a limit on this, ie not
switch to insertion sort if n is larger than 1000 or so, ie

-    if (swap_cnt == 0)
+    if (swap_cnt == 0 && n < 1000)

I'm wondering what the authors were expecting the insertion sort to
handle exactly.  Does anyone have a copy of the paper that's referenced
in the code comment?

/** Qsort routine from Bentley & McIlroy's "Engineering a Sort Function".*/

I tried looking for this at ACM but they seem not to have it.
        regards, tom lane

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From: Simon Riggs
Subject: Re: Separate BLCKSZ for data and logging
From: "Jonah H. Harris"
Subject: Re: qsort, once again