PostgreSQL 8.4.10 Documentation
Prev | Fast Backward |  | Fast Forward | Next

# 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 runtime, 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%';

## Example

     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)

Prev | Home | Next
dblink_disconnect | Up | dblink_exec