A very Strange SQL of PostgreSQL

There is a Strange SQL in our PostgreSQL SERVER, version 8.4. It is looks like a system sql which excuted by PG server ! I have no idea about this sql? Anyone know this ?

--Strange sql

SELECT NULL AS TABLE_CAT, 
       n.nspname AS TABLE_SCHEM, 
       c.relname AS TABLE_NAME,  
       CASE n.nspname ~ '^pg_' OR n.nspname = 'information_schema'  
          WHEN true THEN 
            CASE 
               WHEN n.nspname = 'pg_catalog' OR n.nspname = 'information_schema' THEN 
                 CASE c.relkind   
                   WHEN 'r' THEN 'SYSTEM TABLE'   
                   WHEN 'v' THEN 'SYSTEM VIEW'   
                   WHEN 'i' THEN 'SYSTEM INDEX'   
开发者_StackOverflow社区                   ELSE NULL   
                 END  
               WHEN n.nspname = 'pg_toast' THEN 
                 CASE c.relkind 
                   WHEN 'r' THEN 'SYSTEM TOAST TABLE'   
                   WHEN 'i' THEN 'SYSTEM TOAST INDEX'   
                   ELSE NULL   
                 END  
               ELSE 
                 CASE c.relkind   
                   WHEN 'r' THEN 'TEMPORARY TABLE'   
                   WHEN 'i' THEN 'TEMPORARY INDEX'   
                   ELSE NULL   
                 END  
             END  
             WHEN false THEN 
               CASE c.relkind  
                 WHEN 'r' THEN 'TABLE'  
                 WHEN 'i' THEN 'INDEX'  
                 WHEN 'S' THEN 'SEQUENCE'  
                 WHEN 'v' THEN 'VIEW'  
                 ELSE NULL  
               END  
             ELSE NULL  
           END  AS TABLE_TYPE, 
           d.description AS REMARKS  
      FROM pg_catalog.pg_namespace n, 
           pg_catalog.pg_class c  
 LEFT JOIN pg_catalog.pg_description d ON (c.oid = d.objoid 
                                      AND d.objsubid = 0)  
 LEFT JOIN pg_catalog.pg_class dc ON (d.classoid = dc.oid 
                                 AND dc.relname='pg_class'

It is part of the getTables() implementation in the postgresql JDBC driver.

Update

While Google Code Search was retired a few years after this answer was originally posted, you can see this logic today in the pgjdbc "meta data" sources.

This problem also occures when your application uses some connection manager like c3p0. C3P0 has option preferredTestQuery which defines a query to be executed before acquiring a connection from pool of connection.

If you didn't set this option it would do some weird sql query, otherwise you can tell it to use preferredTestQuery=SELECT 1