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><H1
><A
NAME="SQL-DECLARE"
></A
>DECLARE</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN72823"
></A
><H2
>Name</H2
>DECLARE -- define a cursor</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN72831"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>DECLARE <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> [ BINARY ] [ INSENSITIVE ] [ [ NO ] SCROLL ]
CURSOR [ { WITH | WITHOUT } HOLD ] FOR <TT
CLASS="REPLACEABLE"
><I
>query</I
></TT
></PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN72835"
></A
><H2
>Description</H2
><P
> <TT
CLASS="COMMAND"
>DECLARE</TT
> allows a user to create cursors, which
can be used to retrieve
a small number of rows at a time out of a larger query.
After the cursor is created, rows are fetched from it using
<A
HREF="sql-fetch.html"
>FETCH</A
>.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> This page describes usage of cursors at the SQL command level.
If you are trying to use cursors inside a <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
>
function, the rules are different —
see <A
HREF="plpgsql-cursors.html"
>Section 39.7</A
>.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN72844"
></A
><H2
>Parameters</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
></DT
><DD
><P
> The name of the cursor to be created.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>BINARY</TT
></DT
><DD
><P
> Causes the cursor to return data in binary rather than in text format.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>INSENSITIVE</TT
></DT
><DD
><P
> Indicates that data retrieved from the cursor should be
unaffected by updates to the table(s) underlying the cursor that occur
after the cursor is created. In <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>,
this is the default behavior; so this key word has no
effect and is only accepted for compatibility with the SQL standard.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>SCROLL</TT
><BR><TT
CLASS="LITERAL"
>NO SCROLL</TT
></DT
><DD
><P
><TT
CLASS="LITERAL"
>SCROLL</TT
> specifies that the cursor can be used
to retrieve rows in a nonsequential fashion (e.g.,
backward). Depending upon the complexity of the query's
execution plan, specifying <TT
CLASS="LITERAL"
>SCROLL</TT
> might impose
a performance penalty on the query's execution time.
<TT
CLASS="LITERAL"
>NO SCROLL</TT
> specifies that the cursor cannot be
used to retrieve rows in a nonsequential fashion. The default is to
allow scrolling in some cases; this is not the same as specifying
<TT
CLASS="LITERAL"
>SCROLL</TT
>. See <A
HREF="sql-declare.html#SQL-DECLARE-NOTES"
><I
>Notes</I
></A
> for details.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>WITH HOLD</TT
><BR><TT
CLASS="LITERAL"
>WITHOUT HOLD</TT
></DT
><DD
><P
><TT
CLASS="LITERAL"
>WITH HOLD</TT
> specifies that the cursor can
continue to be used after the transaction that created it
successfully commits. <TT
CLASS="LITERAL"
>WITHOUT HOLD</TT
> specifies
that the cursor cannot be used outside of the transaction that
created it. If neither <TT
CLASS="LITERAL"
>WITHOUT HOLD</TT
> nor
<TT
CLASS="LITERAL"
>WITH HOLD</TT
> is specified, <TT
CLASS="LITERAL"
>WITHOUT
HOLD</TT
> is the default.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>query</I
></TT
></DT
><DD
><P
> A <A
HREF="sql-select.html"
>SELECT</A
> or
<A
HREF="sql-values.html"
>VALUES</A
> command
which will provide the rows to be returned by the cursor.
</P
></DD
></DL
></DIV
><P
> The key words <TT
CLASS="LITERAL"
>BINARY</TT
>,
<TT
CLASS="LITERAL"
>INSENSITIVE</TT
>, and <TT
CLASS="LITERAL"
>SCROLL</TT
> can
appear in any order.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="SQL-DECLARE-NOTES"
></A
><H2
>Notes</H2
><P
> Normal cursors return data in text format, the same as a
<TT
CLASS="COMMAND"
>SELECT</TT
> would produce. The <TT
CLASS="LITERAL"
>BINARY</TT
> option
specifies that the cursor should return data in binary format.
This reduces conversion effort for both the server and client,
at the cost of more programmer effort to deal with platform-dependent
binary data formats.
As an example, if a query returns a value of one from an integer column,
you would get a string of <TT
CLASS="LITERAL"
>1</TT
> with a default cursor,
whereas with a binary cursor you would get
a 4-byte field containing the internal representation of the value
(in big-endian byte order).
</P
><P
> Binary cursors should be used carefully. Many applications,
including <SPAN
CLASS="APPLICATION"
>psql</SPAN
>, are not prepared to
handle binary cursors and expect data to come back in the text
format.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> When the client application uses the <SPAN
CLASS="QUOTE"
>"extended query"</SPAN
> protocol
to issue a <TT
CLASS="COMMAND"
>FETCH</TT
> command, the Bind protocol message
specifies whether data is to be retrieved in text or binary format.
This choice overrides the way that the cursor is defined. The concept
of a binary cursor as such is thus obsolete when using extended query
protocol — any cursor can be treated as either text or binary.
</P
></BLOCKQUOTE
></DIV
><P
> Unless <TT
CLASS="LITERAL"
>WITH HOLD</TT
> is specified, the cursor
created by this command can only be used within the current
transaction. Thus, <TT
CLASS="COMMAND"
>DECLARE</TT
> without <TT
CLASS="LITERAL"
>WITH
HOLD</TT
> is useless outside a transaction block: the cursor would
survive only to the completion of the statement. Therefore
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> reports an error if such a
command is used outside a transaction block.
Use
<A
HREF="sql-begin.html"
>BEGIN</A
> and
<A
HREF="sql-commit.html"
>COMMIT</A
>
(or <A
HREF="sql-rollback.html"
>ROLLBACK</A
>)
to define a transaction block.
</P
><P
> If <TT
CLASS="LITERAL"
>WITH HOLD</TT
> is specified and the transaction
that created the cursor successfully commits, the cursor can
continue to be accessed by subsequent transactions in the same
session. (But if the creating transaction is aborted, the cursor
is removed.) A cursor created with <TT
CLASS="LITERAL"
>WITH HOLD</TT
>
is closed when an explicit <TT
CLASS="COMMAND"
>CLOSE</TT
> command is
issued on it, or the session ends. In the current implementation,
the rows represented by a held cursor are copied into a temporary
file or memory area so that they remain available for subsequent
transactions.
</P
><P
> <TT
CLASS="LITERAL"
>WITH HOLD</TT
> may not be specified when the query
includes <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> or <TT
CLASS="LITERAL"
>FOR SHARE</TT
>.
</P
><P
> The <TT
CLASS="LITERAL"
>SCROLL</TT
> option should be specified when defining a
cursor that will be used to fetch backwards. This is required by
the SQL standard. However, for compatibility with earlier
versions, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will allow
backward fetches without <TT
CLASS="LITERAL"
>SCROLL</TT
>, if the cursor's query
plan is simple enough that no extra overhead is needed to support
it. However, application developers are advised not to rely on
using backward fetches from a cursor that has not been created
with <TT
CLASS="LITERAL"
>SCROLL</TT
>. If <TT
CLASS="LITERAL"
>NO SCROLL</TT
> is
specified, then backward fetches are disallowed in any case.
</P
><P
> Backward fetches are also disallowed when the query
includes <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> or <TT
CLASS="LITERAL"
>FOR SHARE</TT
>; therefore
<TT
CLASS="LITERAL"
>SCROLL</TT
> may not be specified in this case.
</P
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
CLASS="CAUTION"
BORDER="1"
WIDTH="100%"
><TR
><TD
ALIGN="CENTER"
><B
>Caution</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
> Scrollable and <TT
CLASS="LITERAL"
>WITH HOLD</TT
> cursors may give unexpected
results if they invoke any volatile functions (see <A
HREF="xfunc-volatility.html"
>Section 35.6</A
>). When a previously fetched row is
re-fetched, the functions might be re-executed, perhaps leading to
results different from the first time. One workaround for such cases
is to declare the cursor <TT
CLASS="LITERAL"
>WITH HOLD</TT
> and commit the
transaction before reading any rows from it. This will force the
entire output of the cursor to be materialized in temporary storage,
so that volatile functions are executed exactly once for each row.
</P
></TD
></TR
></TABLE
></DIV
><P
> If the cursor's query includes <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> or <TT
CLASS="LITERAL"
>FOR
SHARE</TT
>, then returned rows are locked at the time they are first
fetched, in the same way as for a regular
<A
HREF="sql-select.html"
>SELECT</A
> command with
these options.
In addition, the returned rows will be the most up-to-date versions;
therefore these options provide the equivalent of what the SQL standard
calls a <SPAN
CLASS="QUOTE"
>"sensitive cursor"</SPAN
>. (Specifying <TT
CLASS="LITERAL"
>INSENSITIVE</TT
>
together with <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> or <TT
CLASS="LITERAL"
>FOR SHARE</TT
> is an error.)
</P
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
CLASS="CAUTION"
BORDER="1"
WIDTH="100%"
><TR
><TD
ALIGN="CENTER"
><B
>Caution</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
> It is generally recommended to use <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> if the cursor
is intended to be used with <TT
CLASS="COMMAND"
>UPDATE ... WHERE CURRENT OF</TT
> or
<TT
CLASS="COMMAND"
>DELETE ... WHERE CURRENT OF</TT
>. Using <TT
CLASS="LITERAL"
>FOR UPDATE</TT
>
prevents other sessions from changing the rows between the time they are
fetched and the time they are updated. Without <TT
CLASS="LITERAL"
>FOR UPDATE</TT
>,
a subsequent <TT
CLASS="LITERAL"
>WHERE CURRENT OF</TT
> command will have no effect if
the row was changed since the cursor was created.
</P
><P
> Another reason to use <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> is that without it, a
subsequent <TT
CLASS="LITERAL"
>WHERE CURRENT OF</TT
> might fail if the cursor query
does not meet the SQL standard's rules for being <SPAN
CLASS="QUOTE"
>"simply
updatable"</SPAN
> (in particular, the cursor must reference just one table
and not use grouping or <TT
CLASS="LITERAL"
>ORDER BY</TT
>). Cursors
that are not simply updatable might work, or might not, depending on plan
choice details; so in the worst case, an application might work in testing
and then fail in production.
</P
><P
> The main reason not to use <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> with <TT
CLASS="LITERAL"
>WHERE
CURRENT OF</TT
> is if you need the cursor to be scrollable, or to be
insensitive to the subsequent updates (that is, continue to show the old
data). If this is a requirement, pay close heed to the caveats shown
above.
</P
></TD
></TR
></TABLE
></DIV
><P
> The SQL standard only makes provisions for cursors in embedded
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>. The <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
server does not implement an <TT
CLASS="COMMAND"
>OPEN</TT
> statement for
cursors; a cursor is considered to be open when it is declared.
However, <SPAN
CLASS="APPLICATION"
>ECPG</SPAN
>, the embedded SQL
preprocessor for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, supports
the standard SQL cursor conventions, including those involving
<TT
CLASS="COMMAND"
>DECLARE</TT
> and <TT
CLASS="COMMAND"
>OPEN</TT
> statements.
</P
><P
> You can see all available cursors by querying the <A
HREF="view-pg-cursors.html"
><TT
CLASS="STRUCTNAME"
>pg_cursors</TT
></A
>
system view.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN72976"
></A
><H2
>Examples</H2
><P
> To declare a cursor:
</P><PRE
CLASS="PROGRAMLISTING"
>DECLARE liahona CURSOR FOR SELECT * FROM films;</PRE
><P>
See <A
HREF="sql-fetch.html"
>FETCH</A
> for more
examples of cursor usage.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN72981"
></A
><H2
>Compatibility</H2
><P
> The SQL standard says that it is implementation-dependent whether cursors
are sensitive to concurrent updates of the underlying data by default. In
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, cursors are insensitive by default,
and can be made sensitive by specifying <TT
CLASS="LITERAL"
>FOR UPDATE</TT
>. Other
products may work differently.
</P
><P
> The SQL standard allows cursors only in embedded
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> and in modules. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
permits cursors to be used interactively.
</P
><P
> Binary cursors are a <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
extension.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN72991"
></A
><H2
>See Also</H2
><A
HREF="sql-close.html"
>CLOSE</A
>, <A
HREF="sql-fetch.html"
>FETCH</A
>, <A
HREF="sql-move.html"
>MOVE</A
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