How to Efficiently Aggregate a Single Column in PostgreSQL Queries with Multiple Columns?

How to Aggregate a Single Column in a Query with Multiple Additional Columns

In PostgreSQL, aggregating a single column while including other columns in the query can be cumbersome. However, there are solutions available.

Using Group By for Primary Keys

PostgreSQL 9.1 and later simplifies the process. When aggregating with GROUP BY, you only need to include the primary key of the table. Thus, if foo1 is the primary key of tbl1, your query can be simplified to:

SELECT foo1, foo2, foo3, foo4, foo5, foo6, string_agg(aggregated_field, ', ')
FROM   tbl1
GROUP  BY 1
ORDER  BY foo7, foo8;

Aggregating Before Joining

For queries with multiple tables and relationships, it can be more efficient to aggregate first and join later. For example:

SELECT t1.foo1, t1.foo2, ...
     , t2.bar1, t2.bar2, ...
     , a.aggregated_col 
FROM   tbl1 t1
LEFT   JOIN tbl2 t2 ON ...
...
LEFT   JOIN (
   SELECT some_id, string_agg(agg_col, ', ') AS aggregated_col
   FROM   agg_tbl a ON ...
   GROUP  BY some_id
   ) a ON a.some_id = ?.some_id
ORDER  BY ...

This ensures that aggregation is done only in the relevant part of the query, improving performance.

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