The problem for me was simply finding it again. I’d made a number of searches, including:

• postgres rows to columns
• postgres rows to columns array_to_string
• postgres rows to columns separator
• postgres rows to aggregate
• postgres rows to group
• postgres rows to array

The last one was the key which lead me to stack overflow, and was just what I was looking for.

CREATE AGGREGATE array_accum (anyelement)
(
sfunc = array_append,
stype = anyarray,
initcond = '{}'
);


Once I’ve created that function, I can now do this to turn my rows into a comma separated list:
SELECT array_to_string(array_accum(engine_name), ', ')
FROM search_engine

which turns this:
Bing
Google
into this:
Bing, Google