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Update sequence FAQ items, per suggestion from Pavel Stehule.

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Bruce Momjian 2007-10-09 19:55:36 +00:00
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doc/FAQ
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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Mon Oct 8 23:19:46 EDT 2007
Last updated: Tue Oct 9 15:52:10 EDT 2007
Current maintainer: Bruce Momjian (bruce@momjian.us)
@ -697,28 +697,19 @@
name TEXT
);
See the create_sequence manual page for more information about
sequences.
Automatically created sequence are named <table>_<serialcolumn>_seq,
where table and serialcolumn are the names of the table and SERIAL
column, respectively. See the create_sequence manual page for more
information about sequences.
4.11.2) How do I get the value of a SERIAL insert?
One approach is to retrieve the next SERIAL value from the sequence
object with the nextval() function before inserting and then insert it
explicitly. Using the example table in 4.11.1, an example in a
pseudo-language would look like this:
new_id = execute("SELECT nextval('person_id_seq')");
execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
The simplest way is to retrieve the assigned SERIAL value with
RETURNING. Using the example table in 4.11.1, it would look like this:
INSERT INTO person (name) VALUES ('Blaise Pascal') RETURNING id;
You would then also have the new value stored in new_id for use in
other queries (e.g., as a foreign key to the person table). Note that
the name of the automatically created SEQUENCE object will be named
<table>_< serialcolumn>_seq, where table and serialcolumn are the
names of your table and your SERIAL column, respectively.
Alternatively, you could retrieve the assigned SERIAL value with the
currval() function after it was inserted by default, e.g.,
execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
new_id = execute("SELECT currval('person_id_seq')");
You can also call nextval() and use that value in the INSERT, or call
currval() after the INSERT.
4.11.3) Doesn't currval() lead to a race condition with other users?

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doc/src/FAQ/FAQ.html
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alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Mon Oct 8 23:19:46 EDT 2007</P>
<P>Last updated: Tue Oct 9 15:52:10 EDT 2007</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:bruce@momjian.us">bruce@momjian.us</A>)
@ -916,37 +916,27 @@ length</TD></TR>
);
</PRE>
See the <I>create_sequence</I> manual page for more information
about sequences.
<P>Automatically created sequence are named
&lt;<I>table</I>&gt;_&lt;<I>serialcolumn</I>&gt;_<I>seq</I>, where
<I>table</I> and <I>serialcolumn</I> are the names of the table and
<SMALL>SERIAL</SMALL> column, respectively. See the
<I>create_sequence</I> manual page for more information about
sequences.</P>
<H3 id="item4.11.2">4.11.2) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?</H3>
<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
from the sequence object with the <I>nextval()</I> function
<I>before</I> inserting and then insert it explicitly. Using the
example table in <A href="#item4.11.1">4.11.1</A>, an example in a
pseudo-language would look like this:</P>
<P>The simplest way is to retrieve the assigned <SMALL>SERIAL</SMALL>
value with <SMALL>RETURNING</SMALL>. Using the example table in <A
href="#item4.11.1">4.11.1</A>, it would look like this:</P>
<PRE>
new_id = execute("SELECT nextval('person_id_seq')");
execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
INSERT INTO person (name) VALUES ('Blaise Pascal') RETURNING id;
</PRE>
You would then also have the new value stored in <CODE>new_id</CODE>
for use in other queries (e.g., as a foreign key to the <CODE>person
</CODE> table). Note that the name of the automatically created
<SMALL>SEQUENCE</SMALL> object will be named &lt;<I>table</I>&gt;_&lt;<I>
serialcolumn</I>&gt;_<I>seq</I>, where <I>table</I> and <I>serialcolumn</I>
are the names of your table and your <SMALL>SERIAL</SMALL> column,
respectively.
<P>Alternatively, you could retrieve the assigned <SMALL>SERIAL</SMALL>
value with the <I>currval()</I> function <I>after</I> it was inserted by
default, e.g.,</P>
<PRE>
execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
new_id = execute("SELECT currval('person_id_seq')");
</PRE>
You can also call <I>nextval()</I> and use that value in the
<SMALL>INSERT</SMALL>, or call <I>currval()</I> <I>after</I> the
<SMALL>INSERT</SMALL>.
<H3 id="item4.11.3">4.11.3) Doesn't <I>currval()</I>
lead to a race condition with other users?</H3>