diff options
Diffstat (limited to 'doc/src/sgml/ref/create_sequence.sgml')
| -rw-r--r-- | doc/src/sgml/ref/create_sequence.sgml | 40 |
1 files changed, 36 insertions, 4 deletions
diff --git a/doc/src/sgml/ref/create_sequence.sgml b/doc/src/sgml/ref/create_sequence.sgml index d678bb319c6..2ba60a56d4e 100644 --- a/doc/src/sgml/ref/create_sequence.sgml +++ b/doc/src/sgml/ref/create_sequence.sgml @@ -68,7 +68,7 @@ CREATE SEQUENCE <replaceable class="parameter">seqname</replaceable> The optional clause <option>MINVALUE <replaceable class="parameter">minvalue</replaceable></option> determines the minimum value - a sequence can be. The defaults are 1 and -2147483647 for + a sequence can generate. The defaults are 1 and -2147483647 for ascending and descending sequences, respectively. </PARA> </LISTITEM> @@ -113,7 +113,8 @@ CREATE SEQUENCE <replaceable class="parameter">seqname</replaceable> The <option>CACHE <replaceable class="parameter">cache</replaceable></option> option enables sequence numbers to be preallocated and stored in memory for faster access. The minimum - value is 1 (no cache) and this is also the default. + value is 1 (only one value can be generated at a time, i.e. no cache) + and this is also the default. </PARA> </LISTITEM> </VARLISTENTRY> @@ -223,12 +224,12 @@ CREATE SEQUENCE <replaceable class="parameter">seqname</replaceable> <PARA> CREATE SEQUENCE will enter a new sequence number generator into the current data base. This involves creating and initialising a - new single block + new single-row table with the name <replaceable class="parameter">seqname</replaceable>. The generator will be "owned" by the user issuing the command. </PARA> <para> - After the sequence is created, you may use the function + After a sequence is created, you may use the function <function>nextval(<replaceable class="parameter">seqname</replaceable>)</function> to get a new number from the sequence. The function @@ -245,11 +246,42 @@ The function SELECT * FROM sequence_name; </programlisting> to get the parameters of a sequence. + Aside from fetching the original + parameters, you can use + <programlisting> +SELECT last_value FROM sequence_name; + </programlisting> + to obtain the last value allocated by any backend. + parameters, you can use </para> <para> Low-level locking is used to enable multiple simultaneous calls to a generator. </para> + + <para> + <TITLE> + NOTE: + </TITLE> + 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. + </para> <REFSECT2 ID="R2-SQL-CREATESEQUENCE-3"> <REFSECT2INFO> |