*
*
* IDENTIFICATION
- * $Header: /cvsroot/pgsql/src/backend/nodes/outfuncs.c,v 1.38 1998/07/13 21:27:58 momjian Exp $
+ * $Header: /cvsroot/pgsql/src/backend/nodes/outfuncs.c,v 1.39 1998/07/14 01:45:24 momjian Exp $
*
* NOTES
* Every (plan) node in POSTGRES has an associated "out" routine which
sprintf(buf, " :restypmod %d ", node->restypmod);
appendStringInfo(str, buf);
appendStringInfo(str, " :resname ");
- if (*node->resname)
- sprintf(buf,"\"%s\"", node->resname); /* fix for SELECT col AS "my name" */
- else
- buf[0] = '\0';
+ sprintf(buf,"\"%s\"", node->resname); /* fix for SELECT col AS "my name" */
appendStringInfo(str, buf);
sprintf(buf, " :reskey %d ", node->reskey);
appendStringInfo(str, buf);
appendStringInfo(str, buf);
}
}
-
}
static void
.\" This is -*-nroff-*-
.\" XXX standard disclaimer belongs here....
-.\" $Header: /cvsroot/pgsql/src/man/Attic/create_sequence.l,v 1.4 1998/06/23 17:52:32 momjian Exp $
-.TH "CREATE SEQUENCE" SQL 04/01/97 PostgreSQL PostgreSQL
+.\" $Header: /cvsroot/pgsql/src/man/Attic/create_sequence.l,v 1.5 1998/07/14 01:45:25 momjian Exp $
+.TH "CREATE SEQUENCE" SQL 07/13/98 PostgreSQL PostgreSQL
.SH NAME
create sequence - create a new sequence number generator
.SH SYNOPSIS
.SH DESCRIPTION
.BR "Create sequence"
will enter a new sequence number generator into the current data base.
-Actually, new single block
+Actually, a new single-record
.BR table
with name
.IR seqname
\*(lqowned\*(rq by the user issuing the command.
.PP
The
-.BR "increment"
-is optional clause. Positive value will make ascending sequence,
+.BR increment
+clause is optional. A positive value will make an ascending sequence,
negative - descending. Default value is 1.
.PP
The optional integer
.BR minvalue
-determines the minimum value a sequence can be. Defaults are
+determines the minimum value the sequence can generate. Defaults are
1/-2147483647 for ascending/descending sequences.
.PP
-Use optional integer
+The optional integer
.BR maxvalue
-to determine the maximum value for sequence. Defaults are
+determines the maximum value the sequence can generate. Defaults are
2147483647/-1 for ascending/descending sequences.
.PP
-The optinal
-.BR "start"
-value enables sequence to begin anywhere. Default is
+The optional
+.BR start
+value sets the first value to be generated. Default is
.BR minvalue
for ascending sequences and
.BR maxvalue
.BR cache
option enables sequence numbers to be preallocated and
stored in memory for faster access. The minimum value is 1
-(i.e. - no cache) and it is default.
-.BR NOTE:
-each backend uses own cache to store allocated numbers.
-Cached but not used in current session numbers will be lost.
+(one value will be allocated at a time, i.e., no cache)
+and that is the default. See below for details.
.PP
The optional
.BR cycle
-keyword may be used to enable sequence to continue when the
+keyword may be used to enable the sequence to continue after the
.BR maxvalue/minvalue
has been reached by ascending/descending sequence.
If the limit is reached, the next number generated will be
.BR minvalue/maxvalue
is.
.PP
-After sequence created, You may use function
+After a sequence object has been created, you may use the function
.BR nextval
-with sequence name as argument to get new number from sequence
-specified.
-Function
+with the sequence name as argument to generate a new number from the
+specified sequence.
+.PP
+The function
.BR currval
('sequence_name')
-may be used to determine number returned by last call to
+may be used to re-fetch the number returned by the last call to
.BR nextval
-for specified sequence in current session.
+for the specified sequence in the current session.
+.BR NOTE:
+currval will return an error if nextval has never been called for the
+given sequence in the current backend session. Also beware that it
+does not give the last number ever allocated, only the last one allocated
+by this backend.
.PP
+Use a query like
.nf
-Use query like
-
-select * from <sequence_name>;
-
-to get parameters of a sequence.
+SELECT * FROM <sequence_name>;
.fi
+to get the parameters of a sequence. Aside from fetching the original
+parameters, you can use
+.nf
+SELECT last_value FROM <sequence_name>;
+.fi
+to obtain the last value allocated by any backend.
.PP
-Low-level locking is used to enable multiple simultaneous calls
-to a generator.
+Low-level locking is used to ensure that multiple backends can safely use
+a sequence object concurrently.
+.PP
+.BR NOTE:
+Unexpected results may be obtained if a cache setting greater than one
+is used for a sequence object that will be used concurrently by multiple
+backends. Each backend will allocate "cache" successive sequence values
+during one access to the sequence object and increase the sequence
+object's last_value accordingly. Then, the next cache-1 uses of nextval
+within that backend simply return the preallocated values without touching
+the shared object. So, numbers allocated but not used in the current session
+will be lost. Furthermore, although multiple backends are guaranteed to
+allocate distinct sequence values, the values may be generated out of
+sequence when all the backends are considered. (For example, with a cache
+setting of 10, backend A might reserve values 1..10 and return nextval=1, then
+backend B might reserve values 11..20 and return nextval=11 before backend
+A has generated nextval=2.) Thus, with a cache setting of one it is safe
+to assume that nextval values are generated sequentially; with a cache
+setting greater than one you should only assume that the nextval values
+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.
.PP
.SH EXAMPLES
.nf
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