How to get a result from dynamic SQL in Postgres?

Question

Raw Table for which rule are stored in one table named md_formula , which are used to map in destination table

Drop/Create/Insert for raw_dbs_transactiondetailscash:

DROP TABLE raw_dbs_transactiondetailscash

CREATE TABLE raw_dbs_transactiondetailscash(
    accountnumber VARCHAR(100),
    referencecurrency VARCHAR(100),
    transactiondate datetime)

INSERT INTO raw_dbs_transactiondetailscash(
    accountnumber, referencecurrency, transactiondate)
    SELECT 'XYZ','$','01/01/2016'            

Drop/Create/Insert for md_formula:

DROP TABLE MD_Formula 

CREATE TABLE MD_Formula (
    Format VARCHAR(20),
    tbl_Src VARCHAR(200),
    Col_src VARCHAR(500),
    tbl_Des VARCHAR(200),
    Col_des VARCHAR(100),
    Condition VARCHAR(500) )

INSERT INTO md_formula(format, tbl_src, Col_src, tbl_des,Col_des)    
    SELECT 'Dbs','raw_dbs_transactiondetailscash','accountnumber',
            'normalized_transaction','account_number'
    UNION ALL
    SELECT 'Dbs','raw_dbs_transactiondetailscash','referencecurrency',
            'normalized_transaction','currency'
    UNION ALL
    SELECT 'Dbs','raw_dbs_transactiondetailscash','transactiondate',
            'normalized_transaction','trade_date'

Get the data from raw table stored in md_Formula Via TSQL (Only One Column Selected for Example)

This will actually execute

SELECT accountnumber
FROM raw_dbs_transactiondetailscash

and get set of data from raw_dbs_transactiondetailscash table

DECLARE @sql VARCHAR(100)

SELECT TOP 1 @sql= 'SELECT '+Col_src+ ' FROM '+tbl_Src FROM MD_Formula

EXEC (@sql)

Via Postgres (Dynamic Query is prepared only, how to get data from raw table in dynamic sql is still a question)

This need to execute

SELECT accountnumber,referencecurrency,transactiondate
FROM raw_dbs_transactiondetailscash

and get result

SELECT 'SELECT '|| string_Agg(col_src,',') ||' FROM ' ||  tbl_src FROM md_formula
WHERE format='Dbs'
GROUP BY tbl_src

Solution

For a dynamic query you need to use the 'execute' command.

EXECUTE dynamic-query-string INTO target-variable...

The manual page for this is here: http://www.postgresql.org/docs/current/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN

HTH