How Can I Efficiently Aggregate a Single Column in a Multi-Column Query?

Aggregate Single Column in Multi-Column Query

Overview

Aggregating a single column in a query with numerous other columns can be a challenge, especially when all SELECT and ORDER BY fields must be either aggregated or part of GROUP BY. This verbose query structure can become cumbersome to manage.

Simplified Query for PostgreSQL 9.1

In PostgreSQL 9.1 or later, a simpler solution exists. When using primary keys as GROUP BY criteria, the query can be streamlined:

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

Query with Multiple Tables

For queries involving multiple tables with varying relationships, it's more efficient to aggregate data first and join tables later:

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 ...

By separating aggregation and joining, the majority of the query can avoid unnecessary aggregation, reducing complexity and improving performance.

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