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Am trying to run the following dblink start transaction;

select dblink_connect('host=remote port=5432 dbname=remote_db user=user password=pass');
insert into local_table (column1, column2 )
select * from dblink(`
    select remote_column1 ltrim(remote_column2, ''TEST'') from remote_table
       where remote_column2 ilike ''TEST%''  
) as t(column1 varchar, column2 varchar) ;

now the above works fine, but am encountering a challenge:- I want instead of hardcoding TEST, I want it to be queried from a local table so I would wish to substitute TEST to something like SELECT uniq_id from some_local_table The logic behind it is I want to get records which has prefix matching the result of SELECT uniq_id from some_local_table but trim the prefix when inserting to local_table hope someone understands me

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1 Answer 1

up vote 4 down vote accepted

This should actually work with a correlated subquery:

SELECT dblink_connect(
                'host=remote port=5432 dbname=remote_db user=user password=pass');

INSERT INTO local_table (column1, column2)
FROM   dblink((
          SELECT '
             SELECT remote_column1 ltrim(remote_column2, ''' || uniq_id || ''')
             FROM   remote_table
             WHERE  remote_column2 ILIKE ''' || uniq_id || '%''
             AND    row_counter > ' || coalesce((
                SELECT max(row_counter)
                FROM   yet_another_local_table), -1)
          FROM   some_local_table
          --WHERE <some_condition>
          LIMIT  1  -- make sure it's one row
       ) AS t(column1 varchar, column2 varchar);

SELECT dblink_disconnect();

The meta-levels and layers of single-quotes may be a bit confusing.

This form is vulnerable to SQLi. Make sure nothing bad can come from some_local_table or use quote_literal(uniq_id) in place of plain uniq_id.

In reply to the comment I added a nested correlated subquery to the example. It returns a single row, so it should work fine. Could also be done by left-joining to it in the FROM clause.

Note that I rewrote the expression to make COALESCE() the outer wrapper, so the query won't fail if there are no rows yet_another_local_table. The way you had it would only catch the case where max(row_counter) IS NULL.

Probably easier to read with format() (requires Postgres 9.1+):

INSERT INTO local_table (column1, column2)
FROM   dblink(
         (SELECT format(
            'SELECT remote_column1 ltrim(remote_column2, %1$L)
             FROM   remote_table
             WHERE  remote_column2 ILIKE %2$L
             AND    row_counter > %3s'
            ,uniq_id,  uniq_id || '%'
               SELECT max(row_counter)
               FROM   yet_another_local_table), -1)
          FROM   some_local_table
          --WHERE <some_condition>
          LIMIT  1)
   ) AS t(column1 varchar, column2 varchar);
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+1 for teaching me ilike :) –  tblPhil Jul 28 '12 at 18:32
@Phil: A rather unexpected collateral benefit. :) If you like ILIKE, you may be interested in this related answer. –  Erwin Brandstetter Jul 28 '12 at 18:59
I'm an Oracle guy & would use REGEXP_LIKE(). It's good knowing equivalents :) –  tblPhil Jul 28 '12 at 19:04
I wanted to include another condition after WHERE remote_column2 ilike ''' || uniq_id || '%''' FROM some_local_table i.e and row_counter>'||(select coalesce(max(row_counter),-1) from yet_another_local_table how would I go about that? –  G. Ngotho Jul 28 '12 at 19:44
@user1328181: added to my answer to cover your 2nd question. –  Erwin Brandstetter Jul 28 '12 at 21:44

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