Question

What I’m trying to accomplish is to get aggregated data for all unique combinations of sendercompid, targetcompid, msgtype through all tables in inner SQL.

I expect to have from 20mil to 40mil unique rows in resulting output.

I cannot succeed in running next query on Postgresql 8.3.13:

SELECT 
    sendercompid, targetcompid, count(msgtype), msgtype 
FROM 
    (SELECT table_name 
     FROM information_schema.tables 
     WHERE table_catalog = 'test' 
       AND table_schema = 'msg' 
       AND (table_name like 'fix_aee_20121214%') OR 
           (table_name like 'fix_aee2_20121214%')
   ) 
WHERE 
    (sendercompid LIKE '%201%') OR 
    (targetcompid LIKE '%201%') 
GROUP BY 
    sendercompid, targetcompid, msgtype ;

If this select is being split on 2 : outer and inner, then :
inner will provide list of tables and outer will do select and grouping from each table .

If I run those two SQLs as one, I have an alias error from pgsql db

ERROR: subquery in FROM must have an alias

I tried use alias, but this error not disappear.

Any thoughts what I am missing there?

Thank you.

Answer 1

FROM doesn’t work the way you think it does. A sub-select works like any other query: it produces a set of rows. The outer SELECT works with those rows as though they were a table. There’s no special magic beyond that; it has no idea that the values you’re returning are table names, and won’t treat them as such.

You can probably accomplish what you want using the catalog tables, but that would be complicated and hack-y.

Since your sub-tables appear to be date-based partitions, I think what you really want to use is the partitioning support built into Postgres, described in these docs. Essentially your partitions inherit from a parent table, and you set up range constraints on each child. When you query from the parent table with constraint_exclusion enabled, Postgres automatically selects the appropriate partition.