Update sequence FAQ items, per suggestion from Pavel Stehule.
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Bruce Momjian
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doc/FAQ
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doc/FAQ
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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Mon Oct 8 23:19:46 EDT 2007
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Last updated: Tue Oct 9 15:52:10 EDT 2007
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Current maintainer: Bruce Momjian (bruce@momjian.us)
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@ -697,29 +697,20 @@
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name TEXT
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);
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See the create_sequence manual page for more information about
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sequences.
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Automatically created sequence are named <table>_<serialcolumn>_seq,
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where table and serialcolumn are the names of the table and SERIAL
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column, respectively. See the create_sequence manual page for more
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information about sequences.
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4.11.2) How do I get the value of a SERIAL insert?
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One approach is to retrieve the next SERIAL value from the sequence
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object with the nextval() function before inserting and then insert it
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explicitly. Using the example table in 4.11.1, an example in a
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pseudo-language would look like this:
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new_id = execute("SELECT nextval('person_id_seq')");
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execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
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The simplest way is to retrieve the assigned SERIAL value with
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RETURNING. Using the example table in 4.11.1, it would look like this:
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INSERT INTO person (name) VALUES ('Blaise Pascal') RETURNING id;
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You would then also have the new value stored in new_id for use in
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other queries (e.g., as a foreign key to the person table). Note that
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the name of the automatically created SEQUENCE object will be named
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<table>_< serialcolumn>_seq, where table and serialcolumn are the
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names of your table and your SERIAL column, respectively.
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You can also call nextval() and use that value in the INSERT, or call
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currval() after the INSERT.
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Alternatively, you could retrieve the assigned SERIAL value with the
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currval() function after it was inserted by default, e.g.,
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execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
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new_id = execute("SELECT currval('person_id_seq')");
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4.11.3) Doesn't currval() lead to a race condition with other users?
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No. currval() returns the current value assigned by your session, not
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@ -10,7 +10,7 @@
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alink="#0000ff">
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<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
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<P>Last updated: Mon Oct 8 23:19:46 EDT 2007</P>
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<P>Last updated: Tue Oct 9 15:52:10 EDT 2007</P>
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<P>Current maintainer: Bruce Momjian (<A href=
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"mailto:bruce@momjian.us">bruce@momjian.us</A>)
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@ -916,38 +916,28 @@ length</TD></TR>
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);
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</PRE>
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See the <I>create_sequence</I> manual page for more information
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about sequences.
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<P>Automatically created sequence are named
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<<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
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<I>table</I> and <I>serialcolumn</I> are the names of the table and
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<SMALL>SERIAL</SMALL> column, respectively. See the
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<I>create_sequence</I> manual page for more information about
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sequences.</P>
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<H3 id="item4.11.2">4.11.2) How do I get the value of a
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<SMALL>SERIAL</SMALL> insert?</H3>
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<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
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from the sequence object with the <I>nextval()</I> function
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<I>before</I> inserting and then insert it explicitly. Using the
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example table in <A href="#item4.11.1">4.11.1</A>, an example in a
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pseudo-language would look like this:</P>
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<P>The simplest way is to retrieve the assigned <SMALL>SERIAL</SMALL>
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value with <SMALL>RETURNING</SMALL>. Using the example table in <A
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href="#item4.11.1">4.11.1</A>, it would look like this:</P>
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<PRE>
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new_id = execute("SELECT nextval('person_id_seq')");
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execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
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</PRE>
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You would then also have the new value stored in <CODE>new_id</CODE>
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for use in other queries (e.g., as a foreign key to the <CODE>person
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</CODE> table). Note that the name of the automatically created
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<SMALL>SEQUENCE</SMALL> object will be named <<I>table</I>>_<<I>
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serialcolumn</I>>_<I>seq</I>, where <I>table</I> and <I>serialcolumn</I>
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are the names of your table and your <SMALL>SERIAL</SMALL> column,
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respectively.
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<P>Alternatively, you could retrieve the assigned <SMALL>SERIAL</SMALL>
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value with the <I>currval()</I> function <I>after</I> it was inserted by
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default, e.g.,</P>
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<PRE>
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execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
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new_id = execute("SELECT currval('person_id_seq')");
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INSERT INTO person (name) VALUES ('Blaise Pascal') RETURNING id;
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</PRE>
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You can also call <I>nextval()</I> and use that value in the
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<SMALL>INSERT</SMALL>, or call <I>currval()</I> <I>after</I> the
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<SMALL>INSERT</SMALL>.
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<H3 id="item4.11.3">4.11.3) Doesn't <I>currval()</I>
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lead to a race condition with other users?</H3>
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