Silly bug in pgbench's random number generator
pgbench's random number generator was only generating the first and last value
in the specified range half as often as other values in the range. Not that it
actually matters but it may as well do what it claims. This line has a pretty
long and sordid history with various people tweaking it one way and another.
cvs diff: Diffing contrib/pgbench
Index: contrib/pgbench/pgbench.c
===================================================================
RCS file: /home/stark/src/REPOSITORY/pgsql/contrib/pgbench/pgbench.c,v
retrieving revision 1.66
diff -u -r1.66 pgbench.c
--- contrib/pgbench/pgbench.c 24 May 2007 18:54:10 -0000 1.66
+++ contrib/pgbench/pgbench.c 14 Jun 2007 16:22:19 -0000
@@ -191,7 +191,7 @@
static int
getrand(int min, int max)
{
- return min + (int) (((max - min) * (double) random()) / MAX_RANDOM_VALUE + 0.5);
+ return min + (int) (((max - min + 1) * (double) random()) / MAX_RANDOM_VALUE);
}/* call PQexec() and exit() on failure */
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Gregory Stark wrote:
pgbench's random number generator was only generating the first and last value
in the specified range half as often as other values in the range. Not that it
actually matters but it may as well do what it claims. This line has a pretty
long and sordid history with various people tweaking it one way and another.cvs diff: Diffing contrib/pgbench Index: contrib/pgbench/pgbench.c =================================================================== RCS file: /home/stark/src/REPOSITORY/pgsql/contrib/pgbench/pgbench.c,v retrieving revision 1.66 diff -u -r1.66 pgbench.c --- contrib/pgbench/pgbench.c 24 May 2007 18:54:10 -0000 1.66 +++ contrib/pgbench/pgbench.c 14 Jun 2007 16:22:19 -0000 @@ -191,7 +191,7 @@ static int getrand(int min, int max) { - return min + (int) (((max - min) * (double) random()) / MAX_RANDOM_VALUE + 0.5); + return min + (int) (((max - min + 1) * (double) random()) / MAX_RANDOM_VALUE); }/* call PQexec() and exit() on failure */
I think this line should be altered this way:
return min + (int) (((max - min + 1) * (double) random()) / (MAX_RANDOM_VALUE + 1.0));
eliminating the result of max + 1 in a corner case when random() equals to
MAX_RANDOM_VALUE.
--
Alexey Klyukin http://www.commandprompt.com/
The PostgreSQL Company - Command Prompt, Inc.
This has been saved for the 8.4 release:
http://momjian.postgresql.org/cgi-bin/pgpatches_hold
---------------------------------------------------------------------------
Gregory Stark wrote:
pgbench's random number generator was only generating the first and last value
in the specified range half as often as other values in the range. Not that it
actually matters but it may as well do what it claims. This line has a pretty
long and sordid history with various people tweaking it one way and another.cvs diff: Diffing contrib/pgbench Index: contrib/pgbench/pgbench.c =================================================================== RCS file: /home/stark/src/REPOSITORY/pgsql/contrib/pgbench/pgbench.c,v retrieving revision 1.66 diff -u -r1.66 pgbench.c --- contrib/pgbench/pgbench.c 24 May 2007 18:54:10 -0000 1.66 +++ contrib/pgbench/pgbench.c 14 Jun 2007 16:22:19 -0000 @@ -191,7 +191,7 @@ static int getrand(int min, int max) { - return min + (int) (((max - min) * (double) random()) / MAX_RANDOM_VALUE + 0.5); + return min + (int) (((max - min + 1) * (double) random()) / MAX_RANDOM_VALUE); }/* call PQexec() and exit() on failure */
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com---------------------------(end of broadcast)---------------------------
TIP 7: You can help support the PostgreSQL project by donating at
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://www.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
Uhm, this is a one-line *bug fix*.
"Bruce Momjian" <bruce@momjian.us> writes:
This has been saved for the 8.4 release:
http://momjian.postgresql.org/cgi-bin/pgpatches_hold
---------------------------------------------------------------------------
Gregory Stark wrote:
pgbench's random number generator was only generating the first and last value
in the specified range half as often as other values in the range. Not that it
actually matters but it may as well do what it claims. This line has a pretty
long and sordid history with various people tweaking it one way and another.cvs diff: Diffing contrib/pgbench Index: contrib/pgbench/pgbench.c =================================================================== RCS file: /home/stark/src/REPOSITORY/pgsql/contrib/pgbench/pgbench.c,v retrieving revision 1.66 diff -u -r1.66 pgbench.c --- contrib/pgbench/pgbench.c 24 May 2007 18:54:10 -0000 1.66 +++ contrib/pgbench/pgbench.c 14 Jun 2007 16:22:19 -0000 @@ -191,7 +191,7 @@ static int getrand(int min, int max) { - return min + (int) (((max - min) * (double) random()) / MAX_RANDOM_VALUE + 0.5); + return min + (int) (((max - min + 1) * (double) random()) / MAX_RANDOM_VALUE); }/* call PQexec() and exit() on failure */
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com---------------------------(end of broadcast)---------------------------
TIP 7: You can help support the PostgreSQL project by donating at--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://www.enterprisedb.com+ If your life is a hard drive, Christ can be your backup. +
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Gregory Stark wrote:
Uhm, this is a one-line *bug fix*.
There wasn't agreement on how it should be fixed, it is a minor bug, and
as you said, we should be focusing on the significant outstanding
patches submitted before feature freeze.
---------------------------------------------------------------------------
"Bruce Momjian" <bruce@momjian.us> writes:
This has been saved for the 8.4 release:
http://momjian.postgresql.org/cgi-bin/pgpatches_hold
---------------------------------------------------------------------------
Gregory Stark wrote:
pgbench's random number generator was only generating the first and last value
in the specified range half as often as other values in the range. Not that it
actually matters but it may as well do what it claims. This line has a pretty
long and sordid history with various people tweaking it one way and another.cvs diff: Diffing contrib/pgbench Index: contrib/pgbench/pgbench.c =================================================================== RCS file: /home/stark/src/REPOSITORY/pgsql/contrib/pgbench/pgbench.c,v retrieving revision 1.66 diff -u -r1.66 pgbench.c --- contrib/pgbench/pgbench.c 24 May 2007 18:54:10 -0000 1.66 +++ contrib/pgbench/pgbench.c 14 Jun 2007 16:22:19 -0000 @@ -191,7 +191,7 @@ static int getrand(int min, int max) { - return min + (int) (((max - min) * (double) random()) / MAX_RANDOM_VALUE + 0.5); + return min + (int) (((max - min + 1) * (double) random()) / MAX_RANDOM_VALUE); }/* call PQexec() and exit() on failure */
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com---------------------------(end of broadcast)---------------------------
TIP 7: You can help support the PostgreSQL project by donating at--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://www.enterprisedb.com+ If your life is a hard drive, Christ can be your backup. +
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://www.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
Alexey Klyukin <alexk@commandprompt.com> writes:
Gregory Stark wrote:
pgbench's random number generator was only generating the first and last value
in the specified range half as often as other values in the range.
I think this line should be altered this way:
return min + (int) (((max - min + 1) * (double) random()) / (MAX_RANDOM_VALUE + 1.0));
eliminating the result of max + 1 in a corner case when random() equals to
MAX_RANDOM_VALUE.
Yeah, that looks more correct. Applied.
regards, tom lane