Add :client_id automatic variable for custom pgbench scripts.

This makes it easier to write custom scripts that have different logic for
each client.

Gurjeet Singh, with some changes by me.

contrib/pgbench/pgbench.c

@@ -2544,6 +2544,20 @@ main(int argc, char **argv)
 		}
 	}
 
+	/*
+	 * Define a :client_id variable that is unique per connection. But don't
+	 * override an explicit -D switch.
+	 */
+	if (getVariable(&state[0], "client_id") == NULL)
+	{
+		for (i = 0; i < nclients; i++)
+		{
+			snprintf(val, sizeof(val), "%d", i);
+			if (!putVariable(&state[i], "startup", "client_id", val))
+				exit(1);
+		}
+	}
+
 	if (!is_no_vacuum)
 	{
 		fprintf(stderr, "starting vacuum...");

doc/src/sgml/pgbench.sgml

@@ -600,13 +600,39 @@ pgbench <optional> <replaceable>options</> </optional> <replaceable>dbname</>
    Variables can be set by the command-line <option>-D</> option,
    explained above, or by the meta commands explained below.
    In addition to any variables preset by <option>-D</> command-line options,
-   the variable <literal>scale</> is preset to the current scale factor.
+   there are a few variables that are preset automatically, listed in
+   <xref linkend="pgbench-automatic-variables">. A value specified for these
+   variables using <option>-D</> takes precedence over the automatic presets.
    Once set, a variable's
    value can be inserted into a SQL command by writing
    <literal>:</><replaceable>variablename</>.  When running more than
    one client session, each session has its own set of variables.
   </para>
 
+   <table id="pgbench-automatic-variables">
+    <title>Automatic variables</title>
+    <tgroup cols="2">
+     <thead>
+      <row>
+       <entry>Variable</entry>
+       <entry>Description</entry>
+      </row>
+     </thead>
+
+     <tbody>
+      <row>
+       <entry> <literal>scale</literal> </entry>
+       <entry>current scale factor</entry>
+      </row>
+
+      <row>
+       <entry> <literal>client_id</literal> </entry>
+       <entry>unique number identifying the client session (starts from zero)</entry>
+      </row>
+     </tbody>
+    </tgroup>
+   </table>
+
   <para>
    Script file meta commands begin with a backslash (<literal>\</>).
    Arguments to a meta command are separated by white space.