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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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2 changed files with 26 additions and 45 deletions
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doc/FAQ
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Frequently Asked Questions (FAQ) for PostgreSQL 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) Current maintainer: Bruce Momjian (bruce@momjian.us)
@ -697,29 +697,20 @@
name TEXT name TEXT
); );
See the create_sequence manual page for more information about Automatically created sequence are named <table>_<serialcolumn>_seq,
sequences. 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? 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 The simplest way is to retrieve the assigned SERIAL value with
object with the nextval() function before inserting and then insert it RETURNING. Using the example table in 4.11.1, it would look like this:
explicitly. Using the example table in 4.11.1, an example in a INSERT INTO person (name) VALUES ('Blaise Pascal') RETURNING id;
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')");
You would then also have the new value stored in new_id for use in You can also call nextval() and use that value in the INSERT, or call
other queries (e.g., as a foreign key to the person table). Note that currval() after the INSERT.
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')");
4.11.3) Doesn't currval() lead to a race condition with other users? 4.11.3) Doesn't currval() lead to a race condition with other users?
No. currval() returns the current value assigned by your session, not No. currval() returns the current value assigned by your session, not

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doc/src/FAQ/FAQ.html
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alink="#0000ff"> alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1> <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= <P>Current maintainer: Bruce Momjian (<A href=
"mailto:bruce@momjian.us">bruce@momjian.us</A>) "mailto:bruce@momjian.us">bruce@momjian.us</A>)
@ -916,38 +916,28 @@ length</TD></TR>
); );
</PRE> </PRE>
See the <I>create_sequence</I> manual page for more information <P>Automatically created sequence are named
about sequences. &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 <H3 id="item4.11.2">4.11.2) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?</H3> <SMALL>SERIAL</SMALL> insert?</H3>
<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value <P>The simplest way is to retrieve the assigned <SMALL>SERIAL</SMALL>
from the sequence object with the <I>nextval()</I> function value with <SMALL>RETURNING</SMALL>. Using the example table in <A
<I>before</I> inserting and then insert it explicitly. Using the href="#item4.11.1">4.11.1</A>, it would look like this:</P>
example table in <A href="#item4.11.1">4.11.1</A>, an example in a
pseudo-language would look like this:</P>
<PRE> <PRE>
new_id = execute("SELECT nextval('person_id_seq')"); INSERT INTO person (name) VALUES ('Blaise Pascal') RETURNING id;
execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
</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> </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> <H3 id="item4.11.3">4.11.3) Doesn't <I>currval()</I>
lead to a race condition with other users?</H3> lead to a race condition with other users?</H3>