Why are my function's returned columns concatenated in a PostgreSQL join, and how can I fix it?

Columns Concatenated in Record Returned from Function

When joining data from two or more tables, it's essential to ensure proper segregation of columns to avoid data inconsistencies. However, certain database environments, such as PostgreSQL 9.2.4, may encounter issues where columns returned from a function are concatenated into a single column.

Issue Description

Consider a scenario where a query involves a function that retrieves historical data from a table based on an account ID and a specific date. When the function is directly executed, the results are returned as expected, with each column containing its respective data. However, when the function is used within a larger query, the columns from the function become concatenated into a single column. This can significantly hinder data readability and analysis.

Resolution

To resolve this issue, there are two primary approaches:

Approach 1: Decomposing Rows (Postgres 9.3 or Newer)

Postgres 9.3 and later versions introduced the JOIN LATERAL syntax, which simplifies the process of decomposing rows returned from a function:

SELECT '2014-08-12' AS day, 0 AS inbytes, 0 AS outbytes
      , a.username, a.accountid, a.userid
      , f.*   -- but avoid duplicate column names!
FROM account_tab a
      , account_servicetier_for_day(a.accountid, '2014-08-12') f  -- <-- HERE
WHERE a.isdsl = 1
AND a.dslservicetypeid IS NOT NULL
AND NOT EXISTS (
  SELECT * FROM dailyaccounting_tab
  WHERE day = '2014-08-12'
  AND accountid = a.accountid
)
ORDER BY a.username;

Approach 2: Subquery with Record Decomposition (Postgres 9.2 or Older)

For Postgres versions 9.2 and older, an alternative method is to use a subquery that decomposes the row type:

SELECT '2014-08-12' AS day, 0 AS inbytes, 0 AS outbytes
      , a.username, a.accountid, a.userid
      , (a.rec).*   -- but be wary of duplicate column names!
FROM (
  SELECT *, account_servicetier_for_day(a.accountid, '2014-08-12') AS rec
  FROM account_tab a
  WHERE a.isdsl = 1
  AND a.dslservicetypeid Is Not Null
  AND NOT EXISTS (
    SELECT * FROM dailyaccounting_tab
    WHERE day = '2014-08-12'
    AND accountid = a.accountid
   )
) a
ORDER BY a.username;

By implementing these methods, you can effectively decompose rows returned from functions and ensure accurate data segregation when joining tables in PostgreSQL.

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