diff options
Diffstat (limited to 'doc/src/sgml/ref/create_sequence.sgml')
| -rw-r--r-- | doc/src/sgml/ref/create_sequence.sgml | 42 |
1 files changed, 25 insertions, 17 deletions
diff --git a/doc/src/sgml/ref/create_sequence.sgml b/doc/src/sgml/ref/create_sequence.sgml index 4916b8b649b..718037785c2 100644 --- a/doc/src/sgml/ref/create_sequence.sgml +++ b/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> |