Subquery is faster using a function

I have a long query (~200 lines) that I have embedded in a function:

CREATE FUNCTION spot_rate(base_currency character(3),
                          contra_currency character(3),
                          pricing_date date) RETURNS numeric(20,8)

Whether I run the query directly or the function I get similar results and similar performance. So far so good.

Now I have another long query that looks like:

SELECT x, sum(y * spot_rates.spot)
FROM (SELECT a, b, sum(c) FROM t1 JOIN t2 etc. (6 joins here)) AS table_1,
     (SELECT
        currency,
        spot_rate(currency, 'USD', current_date) AS "spot"
      FROM (SELECT DISTINCT currency FROM table_2) AS "currencies"
     ) AS "spot_rates"
WHERE
     table_1.currency = spot_rates.currency
GROUP BY / ORDER BY

This query runs in 300 ms, which is slowish but fast enough at this stage (and probably makes sense given the number of rows and aggregation operations).

If however I replace spot_rate(currency, 'USD', current_date) by its equivalent query, it runs in 5+ seconds.

Running the subquery alone returns in ~200ms whether I use the function or the equivalent query.

Why would the query run more slowly than the function when used as a subquery?

ps: I hope there is a generic answer to this generic problem - if not I'll post more details but creating a contrived example is not straightforward.

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EDIT: EXPLAIN ANALYZE run on the 2 subqueries and whole queries

• subquery with function: http://ift.tt/1cvVnZA
• subquery with direct query: http://ift.tt/1KRONb0
• whole query with function: http://ift.tt/1cvVpk2
• whole query with direct query: http://ift.tt/1KROMUh
• just the function body, using one set of arguments: http://ift.tt/1cvVofQ