pgbench-seed-1.patch

diff --git a/doc/src/sgml/ref/pgbench.sgml b/doc/src/sgml/ref/pgbench.sgml
index 06cd5db..1908896 100644
--- a/doc/src/sgml/ref/pgbench.sgml
+++ b/doc/src/sgml/ref/pgbench.sgml
@@ -678,6 +678,33 @@ pgbench <optional> <replaceable>options</> </optional> <replaceable>dbname</>
        </para>
       </listitem>
      </varlistentry>
+
+     <varlistentry>
+      <term><option>--random-seed=</><replaceable>SEED</></term>
+      <listitem>
+       <para>
+        Set random generator seed.  This random generator is used to initialize
+        per-thread random generator states.
+        Expected values for <replaceable>SEED</> are: <literal>time</> (the default,
+        the seed is based on the current time) or any unsigned integer value.
+        The random generator is invoked explicitely from a pgbench script
+        (<literal>random...</> functions) or implicitely (for instance option
+        <option>--rate</> uses random to schedule transactions).
+        The random generator seed may also be provided through environment variable
+        <literal>PGBENCH_RANDOM_SEED</>.
+      </para>
+      <para>
+        Setting the seed explicitely allows to reproduce a <command>pgbench</> run
+        exactly, as far as random numbers are concerned.
+        From a statistical viewpoint this is a bad idea because it can hide the
+        performance variability or improve performance unduly, e.g. by hitting
+        the same pages than a previous run.
+        However it may also be of great help for debugging, for instance
+        re-running a tricky case which leads to an error.
+        Use wisely.
+       </para>
+      </listitem>
+     </varlistentry>
     </variablelist>
    </para>
 
diff --git a/src/bin/pgbench/pgbench.c b/src/bin/pgbench/pgbench.c
index 076fbd3..d6db19f 100644
--- a/src/bin/pgbench/pgbench.c
+++ b/src/bin/pgbench/pgbench.c
@@ -434,6 +434,7 @@ usage(void)
 		   "  -U, --username=USERNAME  connect as specified database user\n"
 		 "  -V, --version            output version information, then exit\n"
 		   "  -?, --help               show this help, then exit\n"
+		   "  --random-seed=SEED       set random seed (\"time\", integer)\n"
 		   "\n"
 		   "Report bugs to <pgsql-bugs@postgresql.org>.\n",
 		   progname, progname);
@@ -3258,6 +3259,7 @@ main(int argc, char **argv)
 		{"sampling-rate", required_argument, NULL, 4},
 		{"aggregate-interval", required_argument, NULL, 5},
 		{"progress-timestamp", no_argument, NULL, 6},
+		{"random-seed", required_argument, NULL, 7},
 		{NULL, 0, NULL, 0}
 	};
 
@@ -3292,6 +3294,7 @@ main(int argc, char **argv)
 	PGconn	   *con;
 	PGresult   *res;
 	char	   *env;
+	char	   *seed = NULL;
 
 	char		val[64];
 
@@ -3607,6 +3610,9 @@ main(int argc, char **argv)
 				progress_timestamp = true;
 				benchmarking_option_set = true;
 				break;
+			case 7:
+				seed = pg_strdup(optarg);
+				break;
 			default:
 				fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
 				exit(1);
@@ -3845,7 +3851,25 @@ main(int argc, char **argv)
 
 	/* set random seed */
 	INSTR_TIME_SET_CURRENT(start_time);
-	srandom((unsigned int) INSTR_TIME_GET_MICROSEC(start_time));
+
+	if (seed == NULL)
+		seed = getenv("PGBENCH_RANDOM_SEED");
+
+	if (seed == NULL || strcmp(seed, "time") == 0)
+		srandom((unsigned int) INSTR_TIME_GET_MICROSEC(start_time));
+	else
+	{
+		unsigned int s;
+		fprintf(stderr, "random seed set to '%s'\n", seed);
+		if (sscanf(seed, "%u", &s) != 1)
+		{
+			fprintf(stderr,
+					"error while scanning '%s', expecting an unsigned integer\n",
+					seed);
+			exit(1);
+		}
+		srandom(s);
+	}
 
 	/* set up thread data structures */
 	threads = (TState *) pg_malloc(sizeof(TState) * nthreads);