EXPLAIN EXTENDED

This is a series of articles on how to strip all strings in a set of their longest common prefix and concatenate the results:

• Longest common prefix: SQL Server
• Longest common prefix: PostgreSQL
• Longest common prefix: Oracle
• Longest common prefix: MySQL

Today, I'll show how to do it in PostgreSQL.

A quick reminder of the problem (taken from Stack Overflow):

I have some data:

id	ref
1	3536757616
1	3536757617
1	3536757618
2	3536757628
2	3536757629
2	3536757630

and want to get the result like this:

id	result
1	3536757616/7/8
2	3536757629/28/30

Essentially, the data should be aggregated on id, and the ref's should be concatenated together and separated by a / (slash), but with longest common prefix removed.

Like with SQL Server, it is possible to do this in PostgreSQL using a single SQL query.

But since PostgreSQL offers a nice ability to create custom aggregates, I'll better demonstrate how to solve this task using ones.

In my opinion, custom aggregates fit here just perfectly.

Since PostgreSQL lacks native support for aggregate concatenation, we will need to create two custom aggregates here:
Read the rest of this entry »