~ubuntu-branches/ubuntu/oneiric/postgresql-9.1/oneiric-security : contents of doc/src/sgml/html/contrib-dblink-function.html at revision 14

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><H1
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NAME="CONTRIB-DBLINK-FUNCTION"
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>dblink</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN133739"
></A
><H2
>Name</H2
>dblink&nbsp;--&nbsp;executes a query in a remote database</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN133742"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>dblink(text connname, text sql [, bool fail_on_error]) returns setof record
dblink(text connstr, text sql [, bool fail_on_error]) returns setof record
dblink(text sql [, bool fail_on_error]) returns setof record</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN133744"
></A
><H2
>Description</H2
><P
>    <CODE
CLASS="FUNCTION"
>dblink</CODE
> executes a query (usually a <TT
CLASS="COMMAND"
>SELECT</TT
>,
    but it can be any SQL statement that returns rows) in a remote database.
   </P
><P
>    When two <TT
CLASS="TYPE"
>text</TT
> arguments are given, the first one is first
    looked up as a persistent connection's name; if found, the command
    is executed on that connection.  If not found, the first argument
    is treated as a connection info string as for <CODE
CLASS="FUNCTION"
>dblink_connect</CODE
>,
    and the indicated connection is made just for the duration of this command.
   </P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN133752"
></A
><H2
>Arguments</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="PARAMETER"
>conname</TT
></DT
><DD
><P
>       Name of the connection to use; omit this parameter to use the
       unnamed connection.
      </P
></DD
><DT
><TT
CLASS="PARAMETER"
>connstr</TT
></DT
><DD
><P
>       A connection info string, as previously described for
       <CODE
CLASS="FUNCTION"
>dblink_connect</CODE
>.
      </P
></DD
><DT
><TT
CLASS="PARAMETER"
>sql</TT
></DT
><DD
><P
>       The SQL query that you wish to execute in the remote database,
       for example <TT
CLASS="LITERAL"
>select * from foo</TT
>.
      </P
></DD
><DT
><TT
CLASS="PARAMETER"
>fail_on_error</TT
></DT
><DD
><P
>       If true (the default when omitted) then an error thrown on the
       remote side of the connection causes an error to also be thrown
       locally. If false, the remote error is locally reported as a NOTICE,
       and the function returns no rows.
      </P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN133777"
></A
><H2
>Return Value</H2
><P
>    The function returns the row(s) produced by the query.  Since
    <CODE
CLASS="FUNCTION"
>dblink</CODE
> can be used with any query, it is declared
    to return <TT
CLASS="TYPE"
>record</TT
>, rather than specifying any particular
    set of columns.  This means that you must specify the expected
    set of columns in the calling query &mdash; otherwise
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> would not know what to expect.
    Here is an example:

</P><PRE
CLASS="PROGRAMLISTING"
>SELECT *
    FROM dblink('dbname=mydb', 'select proname, prosrc from pg_proc')
      AS t1(proname name, prosrc text)
    WHERE proname LIKE 'bytea%';</PRE
><P>

    The <SPAN
CLASS="QUOTE"
>"alias"</SPAN
> part of the <TT
CLASS="LITERAL"
>FROM</TT
> clause must
    specify the column names and types that the function will return.
    (Specifying column names in an alias is actually standard SQL
    syntax, but specifying column types is a <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
    extension.)  This allows the system to understand what
    <TT
CLASS="LITERAL"
>*</TT
> should expand to, and what <TT
CLASS="STRUCTNAME"
>proname</TT
>
    in the <TT
CLASS="LITERAL"
>WHERE</TT
> clause refers to, in advance of trying
    to execute the function.  At run time, an error will be thrown
    if the actual query result from the remote database does not
    have the same number of columns shown in the <TT
CLASS="LITERAL"
>FROM</TT
> clause.
    The column names need not match, however, and <CODE
CLASS="FUNCTION"
>dblink</CODE
>
    does not insist on exact type matches either.  It will succeed
    so long as the returned data strings are valid input for the
    column type declared in the <TT
CLASS="LITERAL"
>FROM</TT
> clause.
   </P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN133793"
></A
><H2
>Notes</H2
><P
>    <CODE
CLASS="FUNCTION"
>dblink</CODE
> fetches the entire remote query result before
    returning any of it to the local system.  If the query is expected
    to return a large number of rows, it's better to open it as a cursor
    with <CODE
CLASS="FUNCTION"
>dblink_open</CODE
> and then fetch a manageable number
    of rows at a time.
   </P
><P
>    A convenient way to use <CODE
CLASS="FUNCTION"
>dblink</CODE
> with predetermined
    queries is to create a view.
    This allows the column type information to be buried in the view,
    instead of having to spell it out in every query.  For example,

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE VIEW myremote_pg_proc AS
  SELECT *
    FROM dblink('dbname=postgres', 'select proname, prosrc from pg_proc')
    AS t1(proname name, prosrc text);

SELECT * FROM myremote_pg_proc WHERE proname LIKE 'bytea%';</PRE
><P></P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN133801"
></A
><H2
>Examples</H2
><PRE
CLASS="SCREEN"
>SELECT * FROM dblink('dbname=postgres', 'select proname, prosrc from pg_proc')
  AS t1(proname name, prosrc text) WHERE proname LIKE 'bytea%';
  proname   |   prosrc
------------+------------
 byteacat   | byteacat
 byteaeq    | byteaeq
 bytealt    | bytealt
 byteale    | byteale
 byteagt    | byteagt
 byteage    | byteage
 byteane    | byteane
 byteacmp   | byteacmp
 bytealike  | bytealike
 byteanlike | byteanlike
 byteain    | byteain
 byteaout   | byteaout
(12 rows)

SELECT dblink_connect('dbname=postgres');
 dblink_connect
----------------
 OK
(1 row)

SELECT * FROM dblink('select proname, prosrc from pg_proc')
  AS t1(proname name, prosrc text) WHERE proname LIKE 'bytea%';
  proname   |   prosrc
------------+------------
 byteacat   | byteacat
 byteaeq    | byteaeq
 bytealt    | bytealt
 byteale    | byteale
 byteagt    | byteagt
 byteage    | byteage
 byteane    | byteane
 byteacmp   | byteacmp
 bytealike  | bytealike
 byteanlike | byteanlike
 byteain    | byteain
 byteaout   | byteaout
(12 rows)

SELECT dblink_connect('myconn', 'dbname=regression');
 dblink_connect
----------------
 OK
(1 row)

SELECT * FROM dblink('myconn', 'select proname, prosrc from pg_proc')
  AS t1(proname name, prosrc text) WHERE proname LIKE 'bytea%';
  proname   |   prosrc
------------+------------
 bytearecv  | bytearecv
 byteasend  | byteasend
 byteale    | byteale
 byteagt    | byteagt
 byteage    | byteage
 byteane    | byteane
 byteacmp   | byteacmp
 bytealike  | bytealike
 byteanlike | byteanlike
 byteacat   | byteacat
 byteaeq    | byteaeq
 bytealt    | bytealt
 byteain    | byteain
 byteaout   | byteaout
(14 rows)</PRE
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