PostgreSQL 8.4.22 Documentation | ||||
---|---|---|---|---|
Prev | Fast Backward | Fast Forward | Next |
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
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.
Name of the connection to use; omit this parameter to use the unnamed connection.
A connection info string, as previously described for
dblink_connect
.
The SQL query that you wish to execute in the remote database, for example select * from foo.
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.
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.
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%';
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)