Remove error check that disallowed setval() on a sequence with cache

value greater than one.  The behavior this sought to disallow doesn't
seem any less confusing than the other behaviors of cached sequences.
Improve wording of some error messages, too.
Update documentation accordingly.  Also add an explanation that
aborted transactions do not roll back their nextval() calls; this
seems to be a FAQ, so it ought to be mentioned here...

Author: tglsfdc

doc/src/sgml/ref/create_sequence.sgml

@@ -1,5 +1,5 @@
 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_sequence.sgml,v 1.14 2000/10/05 19:48:17 momjian Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_sequence.sgml,v 1.15 2000/12/08 20:06:58 tgl Exp $
 Postgres documentation
 -->
 
@@ -77,9 +77,9 @@ CREATE SEQUENCE <replaceable class="parameter">seqname</replaceable> [ INCREMENT
       <term><replaceable class="parameter">maxvalue</replaceable></term>
       <listitem>
        <para>
-	Use the optional clause <option>MAXVALUE
-	 <replaceable class="parameter">maxvalue</replaceable></option> to
-	determine the maximum
+	The optional clause <option>MAXVALUE
+	 <replaceable class="parameter">maxvalue</replaceable></option>
+	determines the maximum
 	value for the sequence. The defaults are 2147483647 and -1 for
 	ascending and descending sequences, respectively.
        </para>
@@ -120,15 +120,15 @@ CREATE SEQUENCE <replaceable class="parameter">seqname</replaceable> [ INCREMENT
       <listitem>
        <para>
 	The optional CYCLE keyword may be used to enable the sequence
-	to continue when the
+	to wrap around when the
 	<replaceable class="parameter">maxvalue</replaceable> or
 	<replaceable class="parameter">minvalue</replaceable> has been
 	reached by
 	an ascending or descending sequence respectively. If the limit is
-	reached, the next number generated will be whatever the
+	reached, the next number generated will be the
 	<replaceable class="parameter">minvalue</replaceable> or
-	<replaceable class="parameter">maxvalue</replaceable> is,
-	as appropriate.
+	<replaceable class="parameter">maxvalue</replaceable>,
+	respectively.
        </para>
       </listitem>
      </varlistentry>
@@ -192,7 +192,7 @@ ERROR:  DefineSequence: MINVALUE (<replaceable class="parameter">min</replaceabl
        </computeroutput></term>
       <listitem>
        <para>
-	If the minimum and maximum values are inconsistant.
+	If the minimum and maximum values are inconsistent.
        </para>
       </listitem>
      </varlistentry>
@@ -213,24 +213,24 @@ ERROR:  DefineSequence: MINVALUE (<replaceable class="parameter">min</replaceabl
    into the current data base. This involves creating and initializing a
    new single-row
    table with the name <replaceable class="parameter">seqname</replaceable>.
-   The generator will be "owned" by the user issuing the command.
+   The generator will be owned by the user issuing the command.
   </para>
 
   <para>
    After a sequence is created, you may use the function
-   <function>nextval(<replaceable class="parameter">seqname</replaceable>)</function>
+   <function>nextval('<replaceable class="parameter">seqname</replaceable>')</function>
    to get a new number from the sequence.
    The function
    <function>currval('<replaceable class="parameter">seqname</replaceable>')</function>
    may be used to determine the number returned by the last call to
-   <function>nextval(<replaceable class="parameter">seqname</replaceable>)</function>
+   <function>nextval('<replaceable class="parameter">seqname</replaceable>')</function>
    for the specified sequence in the current session.
    The function
    <function>setval('<replaceable class="parameter">seqname</replaceable>',
     <replaceable class="parameter">newvalue</replaceable>)</function>
    may be used to set the current value of the specified sequence.
    The next call to 
-   <function>nextval(<replaceable class="parameter">seqname</replaceable>)</function>
+   <function>nextval('<replaceable class="parameter">seqname</replaceable>')</function>
    will return the given value plus the sequence increment.
   </para>
 
@@ -241,7 +241,7 @@ ERROR:  DefineSequence: MINVALUE (<replaceable class="parameter">min</replaceabl
 SELECT * FROM <replaceable>seqname</replaceable>;
    </programlisting>
 
-   to get the parameters of a sequence.
+   to examine the parameters of a sequence.
 
    As an alternative to fetching the
    parameters from the original definition as above, you can use
@@ -254,8 +254,13 @@ SELECT last_value FROM <replaceable>seqname</replaceable>;
   </para>
 
   <para>
-   Low-level locking is used to enable multiple simultaneous
-   calls to a generator.
+   To avoid blocking of concurrent transactions
+   that obtain numbers from the same sequence, a nextval operation
+   is never rolled back; that is, once a value has been fetched it is
+   considered used, even if the transaction that did the nextval later
+   aborts.  This means that aborted transactions may leave unused "holes"
+   in the sequence of assigned values.  setval operations are never
+   rolled back, either.
   </para>
 
   <caution>
@@ -279,6 +284,9 @@ SELECT last_value FROM <replaceable>seqname</replaceable>;
     are all distinct, not that they are generated purely sequentially.
     Also, last_value will reflect the latest value reserved by any backend,
     whether or not it has yet been returned by nextval.
+    Another consideration is that a setval executed on such a sequence
+    will not be noticed by other backends until they have used up any
+    preallocated values they have cached.
    </para>
   </caution>
 
@@ -293,7 +301,7 @@ SELECT last_value FROM <replaceable>seqname</replaceable>;
     Use <command>DROP SEQUENCE</command> to remove a sequence.
    </para>
    <para>
-    Each backend uses its own cache to store allocated numbers.
+    Each backend uses its own cache to store preallocated numbers.
     Numbers that are cached but not used in the current session will be
     lost, resulting in "holes" in the sequence.
    </para>

src/backend/commands/sequence.c

@@ -257,7 +257,7 @@ nextval(PG_FUNCTION_ARGS)
 				if (rescnt > 0)
 					break;		/* stop fetching */
 				if (seq->is_cycled != 't')
-					elog(ERROR, "%s.nextval: got MAXVALUE (%d)",
+					elog(ERROR, "%s.nextval: reached MAXVALUE (%d)",
 						 elm->name, maxv);
 				next = minv;
 			}
@@ -273,7 +273,7 @@ nextval(PG_FUNCTION_ARGS)
 				if (rescnt > 0)
 					break;		/* stop fetching */
 				if (seq->is_cycled != 't')
-					elog(ERROR, "%s.nextval: got MINVALUE (%d)",
+					elog(ERROR, "%s.nextval: reached MINVALUE (%d)",
 						 elm->name, minv);
 				next = maxv;
 			}
@@ -371,21 +371,13 @@ do_setval(char *seqname, int32 next, bool iscalled)
 	seq = read_info("setval", elm, &buf);		/* lock page' buffer and
 												 * read tuple */
 
-	if (seq->cache_value != 1)
-	{
-		elog(ERROR, "%s.setval: can't set value of sequence %s, cache != 1",
-			 seqname, seqname);
-	}
-
 	if ((next < seq->min_value) || (next > seq->max_value))
-	{
-		elog(ERROR, "%s.setval: value %d is of of bounds (%d,%d)",
+		elog(ERROR, "%s.setval: value %d is out of bounds (%d,%d)",
 			 seqname, next, seq->min_value, seq->max_value);
-	}
 
 	/* save info in local cache */
 	elm->last = next;			/* last returned number */
-	elm->cached = next;			/* last cached number */
+	elm->cached = next;			/* last cached number (forget cached values) */
 
 	/* save info in sequence relation */
 	START_CRIT_CODE;
@@ -540,7 +532,7 @@ init_sequence(char *caller, char *name)
 	/* Else open and check it */
 	seqrel = heap_openr(name, AccessShareLock);
 	if (seqrel->rd_rel->relkind != RELKIND_SEQUENCE)
-		elog(ERROR, "%s.%s: %s is not sequence !", name, caller, name);
+		elog(ERROR, "%s.%s: %s is not a sequence", name, caller, name);
 
 	if (elm != (SeqTable) NULL)
 	{
@@ -704,7 +696,7 @@ get_param(DefElem *def)
 	if (nodeTag(def->arg) == T_Integer)
 		return intVal(def->arg);
 
-	elog(ERROR, "DefineSequence: \"%s\" is to be integer", def->defname);
+	elog(ERROR, "DefineSequence: \"%s\" value must be integer", def->defname);
 	return -1;
 }