PostgreSQL 9.1.2 Documentation
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# dblink

## Name

dblink -- executes a query in a remote database

## 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

## Description

`dblink` executes a query (usually a `SELECT`, but it can be any SQL statement that returns rows) in a remote database.

When two `text` 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 `dblink_connect`, and the indicated connection is made just for the duration of this command.

## Arguments

`conname`

  Name of the connection to use; omit this parameter to use the unnamed connection.

`connstr`

  A connection info string, as previously described for `dblink_connect`.

`sql`

  The SQL query that you wish to execute in the remote database, for example `select * from foo`.

`fail_on_error`

  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.

## Return Value

The function returns the row(s) produced by the query. Since `dblink` can be used with any query, it is declared to return `record`, rather than specifying any particular set of columns. This means that you must specify the expected set of columns in the calling query — otherwise PostgreSQL would not know what to expect. Here is an example:

    SELECT *
        FROM dblink('dbname=mydb', 'select proname, prosrc from pg_proc')
          AS t1(proname name, prosrc text)
        WHERE proname LIKE 'bytea%';

The "alias" part of the `FROM` 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 PostgreSQL extension.) This allows the system to understand what `*` should expand to, and what `proname` in the `WHERE` 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 `FROM` clause. The column names need not match, however, and `dblink` 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 `FROM` clause.

## Notes

`dblink` 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 `dblink_open` and then fetch a manageable number of rows at a time.

A convenient way to use `dblink` 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,

    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%';

## Examples

    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)

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