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Use xreflabel attributes instead of endterm attributes to control the appearance of links to subsections of SQL command reference pages. This is simpler, it matches what we do elsewhere (e.g. for GUC variables), and it doesn't draw "Unresolved ID reference" warnings from the PDF toolchain. Fix some places where the text was absolutely dependent on an <xref> rendering exactly so, by using a <link> around the required text instead. At least one of those spots had already been turned into bad grammar by subsequent changes, and the whole idea is just too fragile for my taste. <xref> does NOT have fixed output, don't write as if it does. Consistently include a page-level link in cross-man-page references, because otherwise they are useless/nonsensical in man-page output. Likewise, be consistent about mentioning "below" or "above" in same-page references; we were doing that in about 90% of the cases, but now it's 100%. Also get rid of another nonfunctional-in-PDF idea, of making cross-references to functions by sticking ID tags on <row> constructs. We can put the IDs on <indexterm>s instead --- which is probably not any more sensible in abstract terms, but it works where the other doesn't. (There is talk of attaching cross-reference IDs to most or all of the docs' function descriptions, but for now I just fixed the two that exist.) Discussion: https://postgr.es/m/14480.1589154358@sss.pgh.pa.us
351 lines
13 KiB
Plaintext
351 lines
13 KiB
Plaintext
<!--
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doc/src/sgml/ref/declare.sgml
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PostgreSQL documentation
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-->
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<refentry id="sql-declare">
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<indexterm zone="sql-declare">
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<primary>DECLARE</primary>
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</indexterm>
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<indexterm zone="sql-declare">
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<primary>cursor</primary>
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<secondary>DECLARE</secondary>
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</indexterm>
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<refmeta>
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<refentrytitle>DECLARE</refentrytitle>
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<manvolnum>7</manvolnum>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>DECLARE</refname>
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<refpurpose>define a cursor</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<synopsis>
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DECLARE <replaceable class="parameter">name</replaceable> [ BINARY ] [ INSENSITIVE ] [ [ NO ] SCROLL ]
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CURSOR [ { WITH | WITHOUT } HOLD ] FOR <replaceable class="parameter">query</replaceable>
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</synopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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<command>DECLARE</command> allows a user to create cursors, which
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can be used to retrieve
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a small number of rows at a time out of a larger query.
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After the cursor is created, rows are fetched from it using
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<xref linkend="sql-fetch"/>.
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</para>
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<note>
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<para>
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This page describes usage of cursors at the SQL command level.
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If you are trying to use cursors inside a <application>PL/pgSQL</application>
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function, the rules are different —
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see <xref linkend="plpgsql-cursors"/>.
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</para>
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</note>
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</refsect1>
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<refsect1>
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<title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><replaceable class="parameter">name</replaceable></term>
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<listitem>
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<para>
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The name of the cursor to be created.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>BINARY</literal></term>
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<listitem>
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<para>
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Causes the cursor to return data in binary rather than in text format.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>INSENSITIVE</literal></term>
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<listitem>
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<para>
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Indicates that data retrieved from the cursor should be
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unaffected by updates to the table(s) underlying the cursor that occur
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after the cursor is created. In <productname>PostgreSQL</productname>,
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this is the default behavior; so this key word has no
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effect and is only accepted for compatibility with the SQL standard.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>SCROLL</literal></term>
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<term><literal>NO SCROLL</literal></term>
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<listitem>
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<para><literal>SCROLL</literal> specifies that the cursor can be used
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to retrieve rows in a nonsequential fashion (e.g.,
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backward). Depending upon the complexity of the query's
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execution plan, specifying <literal>SCROLL</literal> might impose
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a performance penalty on the query's execution time.
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<literal>NO SCROLL</literal> specifies that the cursor cannot be
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used to retrieve rows in a nonsequential fashion. The default is to
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allow scrolling in some cases; this is not the same as specifying
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<literal>SCROLL</literal>. See <xref linkend="sql-declare-notes"/>
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below for details.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>WITH HOLD</literal></term>
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<term><literal>WITHOUT HOLD</literal></term>
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<listitem>
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<para><literal>WITH HOLD</literal> specifies that the cursor can
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continue to be used after the transaction that created it
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successfully commits. <literal>WITHOUT HOLD</literal> specifies
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that the cursor cannot be used outside of the transaction that
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created it. If neither <literal>WITHOUT HOLD</literal> nor
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<literal>WITH HOLD</literal> is specified, <literal>WITHOUT
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HOLD</literal> is the default.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">query</replaceable></term>
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<listitem>
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<para>
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A <xref linkend="sql-select"/> or
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<xref linkend="sql-values"/> command
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which will provide the rows to be returned by the cursor.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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The key words <literal>BINARY</literal>,
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<literal>INSENSITIVE</literal>, and <literal>SCROLL</literal> can
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appear in any order.
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</para>
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</refsect1>
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<refsect1 id="sql-declare-notes" xreflabel="Notes">
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<title>Notes</title>
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<para>
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Normal cursors return data in text format, the same as a
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<command>SELECT</command> would produce. The <literal>BINARY</literal> option
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specifies that the cursor should return data in binary format.
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This reduces conversion effort for both the server and client,
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at the cost of more programmer effort to deal with platform-dependent
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binary data formats.
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As an example, if a query returns a value of one from an integer column,
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you would get a string of <literal>1</literal> with a default cursor,
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whereas with a binary cursor you would get
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a 4-byte field containing the internal representation of the value
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(in big-endian byte order).
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</para>
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<para>
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Binary cursors should be used carefully. Many applications,
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including <application>psql</application>, are not prepared to
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handle binary cursors and expect data to come back in the text
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format.
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</para>
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<note>
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<para>
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When the client application uses the <quote>extended query</quote> protocol
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to issue a <command>FETCH</command> command, the Bind protocol message
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specifies whether data is to be retrieved in text or binary format.
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This choice overrides the way that the cursor is defined. The concept
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of a binary cursor as such is thus obsolete when using extended query
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protocol — any cursor can be treated as either text or binary.
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</para>
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</note>
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<para>
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Unless <literal>WITH HOLD</literal> is specified, the cursor
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created by this command can only be used within the current
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transaction. Thus, <command>DECLARE</command> without <literal>WITH
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HOLD</literal> is useless outside a transaction block: the cursor would
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survive only to the completion of the statement. Therefore
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<productname>PostgreSQL</productname> reports an error if such a
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command is used outside a transaction block.
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Use
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<xref linkend="sql-begin"/> and
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<xref linkend="sql-commit"/>
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(or <xref linkend="sql-rollback"/>)
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to define a transaction block.
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</para>
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<para>
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If <literal>WITH HOLD</literal> is specified and the transaction
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that created the cursor successfully commits, the cursor can
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continue to be accessed by subsequent transactions in the same
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session. (But if the creating transaction is aborted, the cursor
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is removed.) A cursor created with <literal>WITH HOLD</literal>
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is closed when an explicit <command>CLOSE</command> command is
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issued on it, or the session ends. In the current implementation,
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the rows represented by a held cursor are copied into a temporary
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file or memory area so that they remain available for subsequent
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transactions.
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</para>
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<para>
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<literal>WITH HOLD</literal> may not be specified when the query
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includes <literal>FOR UPDATE</literal> or <literal>FOR SHARE</literal>.
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</para>
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<para>
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The <literal>SCROLL</literal> option should be specified when defining a
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cursor that will be used to fetch backwards. This is required by
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the SQL standard. However, for compatibility with earlier
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versions, <productname>PostgreSQL</productname> will allow
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backward fetches without <literal>SCROLL</literal>, if the cursor's query
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plan is simple enough that no extra overhead is needed to support
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it. However, application developers are advised not to rely on
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using backward fetches from a cursor that has not been created
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with <literal>SCROLL</literal>. If <literal>NO SCROLL</literal> is
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specified, then backward fetches are disallowed in any case.
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</para>
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<para>
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Backward fetches are also disallowed when the query
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includes <literal>FOR UPDATE</literal> or <literal>FOR SHARE</literal>; therefore
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<literal>SCROLL</literal> may not be specified in this case.
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</para>
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<caution>
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<para>
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Scrollable and <literal>WITH HOLD</literal> cursors may give unexpected
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results if they invoke any volatile functions (see <xref
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linkend="xfunc-volatility"/>). When a previously fetched row is
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re-fetched, the functions might be re-executed, perhaps leading to
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results different from the first time. One workaround for such cases
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is to declare the cursor <literal>WITH HOLD</literal> and commit the
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transaction before reading any rows from it. This will force the
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entire output of the cursor to be materialized in temporary storage,
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so that volatile functions are executed exactly once for each row.
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</para>
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</caution>
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<para>
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If the cursor's query includes <literal>FOR UPDATE</literal> or <literal>FOR
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SHARE</literal>, then returned rows are locked at the time they are first
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fetched, in the same way as for a regular
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<xref linkend="sql-select"/> command with
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these options.
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In addition, the returned rows will be the most up-to-date versions;
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therefore these options provide the equivalent of what the SQL standard
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calls a <quote>sensitive cursor</quote>. (Specifying <literal>INSENSITIVE</literal>
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together with <literal>FOR UPDATE</literal> or <literal>FOR SHARE</literal> is an error.)
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</para>
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<caution>
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<para>
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It is generally recommended to use <literal>FOR UPDATE</literal> if the cursor
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is intended to be used with <command>UPDATE ... WHERE CURRENT OF</command> or
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<command>DELETE ... WHERE CURRENT OF</command>. Using <literal>FOR UPDATE</literal>
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prevents other sessions from changing the rows between the time they are
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fetched and the time they are updated. Without <literal>FOR UPDATE</literal>,
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a subsequent <literal>WHERE CURRENT OF</literal> command will have no effect if
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the row was changed since the cursor was created.
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</para>
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<para>
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Another reason to use <literal>FOR UPDATE</literal> is that without it, a
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subsequent <literal>WHERE CURRENT OF</literal> might fail if the cursor query
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does not meet the SQL standard's rules for being <quote>simply
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updatable</quote> (in particular, the cursor must reference just one table
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and not use grouping or <literal>ORDER BY</literal>). Cursors
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that are not simply updatable might work, or might not, depending on plan
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choice details; so in the worst case, an application might work in testing
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and then fail in production. If <literal>FOR UPDATE</literal> is
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specified, the cursor is guaranteed to be updatable.
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</para>
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<para>
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The main reason not to use <literal>FOR UPDATE</literal> with <literal>WHERE
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CURRENT OF</literal> is if you need the cursor to be scrollable, or to be
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insensitive to the subsequent updates (that is, continue to show the old
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data). If this is a requirement, pay close heed to the caveats shown
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above.
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</para>
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</caution>
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<para>
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The SQL standard only makes provisions for cursors in embedded
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<acronym>SQL</acronym>. The <productname>PostgreSQL</productname>
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server does not implement an <command>OPEN</command> statement for
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cursors; a cursor is considered to be open when it is declared.
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However, <application>ECPG</application>, the embedded SQL
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preprocessor for <productname>PostgreSQL</productname>, supports
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the standard SQL cursor conventions, including those involving
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<command>DECLARE</command> and <command>OPEN</command> statements.
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</para>
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<para>
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You can see all available cursors by querying the <link
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linkend="view-pg-cursors"><structname>pg_cursors</structname></link>
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system view.
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</para>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<para>
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To declare a cursor:
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<programlisting>
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DECLARE liahona CURSOR FOR SELECT * FROM films;
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</programlisting>
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See <xref linkend="sql-fetch"/> for more
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examples of cursor usage.
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</para>
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</refsect1>
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<refsect1>
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<title>Compatibility</title>
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<para>
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The SQL standard says that it is implementation-dependent whether cursors
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are sensitive to concurrent updates of the underlying data by default. In
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<productname>PostgreSQL</productname>, cursors are insensitive by default,
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and can be made sensitive by specifying <literal>FOR UPDATE</literal>. Other
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products may work differently.
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</para>
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<para>
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The SQL standard allows cursors only in embedded
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<acronym>SQL</acronym> and in modules. <productname>PostgreSQL</productname>
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permits cursors to be used interactively.
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</para>
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<para>
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Binary cursors are a <productname>PostgreSQL</productname>
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extension.
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</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<simplelist type="inline">
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<member><xref linkend="sql-close"/></member>
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<member><xref linkend="sql-fetch"/></member>
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<member><xref linkend="sql-move"/></member>
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</simplelist>
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</refsect1>
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</refentry>
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