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postgres/doc/src/sgml/ref/fetch.sgml
Peter Eisentraut 9081bddbd7 Improve <xref> vs. <command> formatting in the documentation 
SQL commands are generally marked up as <command>, except when a link
to a reference page is used using <xref>.  But the latter doesn't
create monospace markup, so this looks strange especially when a
paragraph contains a mix of links and non-links.

We considered putting <command> in the <refentrytitle> on the target
side, but that creates some formatting side effects elsewhere.
Generally, it seems safer to solve this on the link source side.

We can't put the <xref> inside the <command>; the DTD doesn't allow
this.  DocBook 5 would allow the <command> to have the linkend
attribute itself, but we are not there yet.

So to solve this for now, convert the <xref>s to <link> plus
<command>.  This gives the correct look and also gives some more
flexibility what we can put into the link text (e.g., subcommands or
other clauses).  In the future, these could then be converted to
DocBook 5 style.

I haven't converted absolutely all xrefs to SQL command reference
pages, only those where we care about the appearance of the link text
or where it was otherwise appropriate to make the appearance match a
bit better.  Also in some cases, the links where repetitive, so in
those cases the links where just removed and replaced by a plain
<command>.  In cases where we just want the link and don't
specifically care about the generated link text (typically phrased
"for further information see <xref ...>") the xref is kept.

Reported-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://www.postgresql.org/message-id/flat/87o8pco34z.fsf@wibble.ilmari.org
2020-10-03 16:40:02 +02:00

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<!--
doc/src/sgml/ref/fetch.sgml
PostgreSQL documentation
-->
<refentry id="sql-fetch">
<indexterm zone="sql-fetch">
<primary>FETCH</primary>
</indexterm>
<indexterm zone="sql-fetch">
<primary>cursor</primary>
<secondary>FETCH</secondary>
</indexterm>
<refmeta>
<refentrytitle>FETCH</refentrytitle>
<manvolnum>7</manvolnum>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>FETCH</refname>
<refpurpose>retrieve rows from a query using a cursor</refpurpose>
</refnamediv>
<refsynopsisdiv>
<!-- Note the "direction" bit is also in ref/move.sgml -->
<synopsis>
FETCH [ <replaceable class="parameter">direction</replaceable> [ FROM | IN ] ] <replaceable class="parameter">cursor_name</replaceable>
<phrase>where <replaceable class="parameter">direction</replaceable> can be empty or one of:</phrase>
NEXT
PRIOR
FIRST
LAST
ABSOLUTE <replaceable class="parameter">count</replaceable>
RELATIVE <replaceable class="parameter">count</replaceable>
<replaceable class="parameter">count</replaceable>
ALL
FORWARD
FORWARD <replaceable class="parameter">count</replaceable>
FORWARD ALL
BACKWARD
BACKWARD <replaceable class="parameter">count</replaceable>
BACKWARD ALL
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<command>FETCH</command> retrieves rows using a previously-created cursor.
</para>
<para>
A cursor has an associated position, which is used by
<command>FETCH</command>. The cursor position can be before the first row of the
query result, on any particular row of the result, or after the last row
of the result. When created, a cursor is positioned before the first row.
After fetching some rows, the cursor is positioned on the row most recently
retrieved. If <command>FETCH</command> runs off the end of the available rows
then the cursor is left positioned after the last row, or before the first
row if fetching backward. <command>FETCH ALL</command> or <command>FETCH BACKWARD
ALL</command> will always leave the cursor positioned after the last row or before
the first row.
</para>
<para>
The forms <literal>NEXT</literal>, <literal>PRIOR</literal>, <literal>FIRST</literal>,
<literal>LAST</literal>, <literal>ABSOLUTE</literal>, <literal>RELATIVE</literal> fetch
a single row after moving the cursor appropriately. If there is no
such row, an empty result is returned, and the cursor is left
positioned before the first row or after the last row as
appropriate.
</para>
<para>
The forms using <literal>FORWARD</literal> and <literal>BACKWARD</literal>
retrieve the indicated number of rows moving in the forward or
backward direction, leaving the cursor positioned on the
last-returned row (or after/before all rows, if the <replaceable
class="parameter">count</replaceable> exceeds the number of rows
available).
</para>
<para>
<literal>RELATIVE 0</literal>, <literal>FORWARD 0</literal>, and
<literal>BACKWARD 0</literal> all request fetching the current row without
moving the cursor, that is, re-fetching the most recently fetched
row. This will succeed unless the cursor is positioned before the
first row or after the last row; in which case, no row is returned.
</para>
<note>
<para>
This page describes usage of cursors at the SQL command level.
If you are trying to use cursors inside a <application>PL/pgSQL</application>
function, the rules are different &mdash;
see <xref linkend="plpgsql-cursor-using"/>.
</para>
</note>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><replaceable class="parameter">direction</replaceable></term>
<listitem>
<para><replaceable class="parameter">direction</replaceable> defines
the fetch direction and number of rows to fetch. It can be one
of the following:
<variablelist>
<varlistentry>
<term><literal>NEXT</literal></term>
<listitem>
<para>
Fetch the next row. This is the default if <replaceable
class="parameter">direction</replaceable> is omitted.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PRIOR</literal></term>
<listitem>
<para>
Fetch the prior row.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>FIRST</literal></term>
<listitem>
<para>
Fetch the first row of the query (same as <literal>ABSOLUTE 1</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>LAST</literal></term>
<listitem>
<para>
Fetch the last row of the query (same as <literal>ABSOLUTE -1</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>ABSOLUTE <replaceable class="parameter">count</replaceable></literal></term>
<listitem>
<para>
Fetch the <replaceable
class="parameter">count</replaceable>'th row of the query,
or the <literal>abs(<replaceable
class="parameter">count</replaceable>)</literal>'th row from
the end if <replaceable
class="parameter">count</replaceable> is negative. Position
before first row or after last row if <replaceable
class="parameter">count</replaceable> is out of range; in
particular, <literal>ABSOLUTE 0</literal> positions before
the first row.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>RELATIVE <replaceable class="parameter">count</replaceable></literal></term>
<listitem>
<para>
Fetch the <replaceable
class="parameter">count</replaceable>'th succeeding row, or
the <literal>abs(<replaceable
class="parameter">count</replaceable>)</literal>'th prior
row if <replaceable class="parameter">count</replaceable> is
negative. <literal>RELATIVE 0</literal> re-fetches the
current row, if any.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">count</replaceable></term>
<listitem>
<para>
Fetch the next <replaceable
class="parameter">count</replaceable> rows (same as
<literal>FORWARD <replaceable
class="parameter">count</replaceable></literal>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>ALL</literal></term>
<listitem>
<para>
Fetch all remaining rows (same as <literal>FORWARD ALL</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>FORWARD</literal></term>
<listitem>
<para>
Fetch the next row (same as <literal>NEXT</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>FORWARD <replaceable class="parameter">count</replaceable></literal></term>
<listitem>
<para>
Fetch the next <replaceable
class="parameter">count</replaceable> rows.
<literal>FORWARD 0</literal> re-fetches the current row.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>FORWARD ALL</literal></term>
<listitem>
<para>
Fetch all remaining rows.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>BACKWARD</literal></term>
<listitem>
<para>
Fetch the prior row (same as <literal>PRIOR</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>BACKWARD <replaceable class="parameter">count</replaceable></literal></term>
<listitem>
<para>
Fetch the prior <replaceable
class="parameter">count</replaceable> rows (scanning
backwards). <literal>BACKWARD 0</literal> re-fetches the
current row.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>BACKWARD ALL</literal></term>
<listitem>
<para>
Fetch all prior rows (scanning backwards).
</para>
</listitem>
</varlistentry>
</variablelist></para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">count</replaceable></term>
<listitem>
<para><replaceable class="parameter">count</replaceable> is a
possibly-signed integer constant, determining the location or
number of rows to fetch. For <literal>FORWARD</literal> and
<literal>BACKWARD</literal> cases, specifying a negative <replaceable
class="parameter">count</replaceable> is equivalent to changing
the sense of <literal>FORWARD</literal> and <literal>BACKWARD</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">cursor_name</replaceable></term>
<listitem>
<para>
An open cursor's name.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Outputs</title>
<para>
On successful completion, a <command>FETCH</command> command returns a command
tag of the form
<screen>
FETCH <replaceable class="parameter">count</replaceable>
</screen>
The <replaceable class="parameter">count</replaceable> is the number
of rows fetched (possibly zero). Note that in
<application>psql</application>, the command tag will not actually be
displayed, since <application>psql</application> displays the fetched
rows instead.
</para>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
The cursor should be declared with the <literal>SCROLL</literal>
option if one intends to use any variants of <command>FETCH</command>
other than <command>FETCH NEXT</command> or <command>FETCH FORWARD</command> with
a positive count. For simple queries
<productname>PostgreSQL</productname> will allow backwards fetch
from cursors not declared with <literal>SCROLL</literal>, but this
behavior is best not relied on. If the cursor is declared with
<literal>NO SCROLL</literal>, no backward fetches are allowed.
</para>
<para>
<literal>ABSOLUTE</literal> fetches are not any faster than
navigating to the desired row with a relative move: the underlying
implementation must traverse all the intermediate rows anyway.
Negative absolute fetches are even worse: the query must be read to
the end to find the last row, and then traversed backward from
there. However, rewinding to the start of the query (as with
<literal>FETCH ABSOLUTE 0</literal>) is fast.
</para>
<para>
<link linkend="sql-declare"><command>DECLARE</command></link>
is used to define a cursor. Use
<link linkend="sql-move"><command>MOVE</command></link>
to change cursor position without retrieving data.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
The following example traverses a table using a cursor:
<programlisting>
BEGIN WORK;
-- Set up a cursor:
DECLARE liahona SCROLL CURSOR FOR SELECT * FROM films;
-- Fetch the first 5 rows in the cursor liahona:
FETCH FORWARD 5 FROM liahona;
code | title | did | date_prod | kind | len
-------+-------------------------+-----+------------+----------+-------
BL101 | The Third Man | 101 | 1949-12-23 | Drama | 01:44
BL102 | The African Queen | 101 | 1951-08-11 | Romantic | 01:43
JL201 | Une Femme est une Femme | 102 | 1961-03-12 | Romantic | 01:25
P_301 | Vertigo | 103 | 1958-11-14 | Action | 02:08
P_302 | Becket | 103 | 1964-02-03 | Drama | 02:28
-- Fetch the previous row:
FETCH PRIOR FROM liahona;
code | title | did | date_prod | kind | len
-------+---------+-----+------------+--------+-------
P_301 | Vertigo | 103 | 1958-11-14 | Action | 02:08
-- Close the cursor and end the transaction:
CLOSE liahona;
COMMIT WORK;
</programlisting></para>
</refsect1>
<refsect1>
<title>Compatibility</title>
<para>
The SQL standard defines <command>FETCH</command> for use in
embedded SQL only. The variant of <command>FETCH</command>
described here returns the data as if it were a
<command>SELECT</command> result rather than placing it in host
variables. Other than this point, <command>FETCH</command> is
fully upward-compatible with the SQL standard.
</para>
<para>
The <command>FETCH</command> forms involving
<literal>FORWARD</literal> and <literal>BACKWARD</literal>, as well
as the forms <literal>FETCH <replaceable
class="parameter">count</replaceable></literal> and <literal>FETCH
ALL</literal>, in which <literal>FORWARD</literal> is implicit, are
<productname>PostgreSQL</productname> extensions.
</para>
<para>
The SQL standard allows only <literal>FROM</literal> preceding the cursor
name; the option to use <literal>IN</literal>, or to leave them out altogether, is
an extension.
</para>
</refsect1>
<refsect1>
<title>See Also</title>
<simplelist type="inline">
<member><xref linkend="sql-close"/></member>
<member><xref linkend="sql-declare"/></member>
<member><xref linkend="sql-move"/></member>
</simplelist>
</refsect1>
</refentry>
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