Question

Suppose I have a table called Transaction and another table called Price. Price holds the prices for given funds at different dates. Each fund will have prices added at various dates, but they won’t have prices at all possible dates. So for fund XYZ I may have prices for the 1 May, 7 May and 13 May and fund ABC may have prices at 3 May, 9 May and 11 May.

So now I’m looking at the price that was prevailing for a fund at the date of a transaction. The transaction was for fund XYZ on 10 May. What I want, is the latest known price on that day, which will be the price for 7 May.

Here’s the code:

select d.TransactionID, d.FundCode, d.TransactionDate, v.OfferPrice
from Transaction d
    inner join Price v
        on v.FundCode = d.FundCode
        and v.PriceDate = (
            select max(PriceDate)
            from Price
            where FundCode = v.FundCode
            /* */ and PriceDate < d.TransactionDate 
        )

It works, but it is very slow (several minutes in real world use). If I remove the line with the leading comment, the query is very quick (2 seconds or so) but it then uses the latest price per fund, which is wrong.

The bad part is that the price table is minuscule compared to some of the other tables we use, and it isn’t clear to me why it is so slow. I suspect the offending line forces SQL Server to process a Cartesian product, but I don’t know how to avoid it.

I keep hoping to find a more efficient way to do this, but it has so far escaped me. Any ideas?

Answer 1

You don’t specify the version of SQL Server you’re using, but if you are using a version with support for ranking functions and CTE queries I think you’ll find this quite a bit more performant than using a correlated subquery within your join statement.

It should be very similar in performance to Andriy’s queries. Depending on the exact index topography of your tables, one approach might be slightly faster than another.

I tend to like CTE-based approaches because the resulting code is quite a bit more readable (in my opinion). Hope this helps!

;WITH set_gen (TransactionID, OfferPrice, Match_val)
AS
(
    SELECT d.TransactionID, v.OfferPrice, ROW_NUMBER() OVER(PARTITION BY d.TransactionID ORDER BY v.PriceDate ASC) AS Match_val
    FROM Transaction d
        INNER JOIN Price v
            ON v.FundCode = d.FundCode
    WHERE v.PriceDate <= d.TransactionDate
)
SELECT sg.TransactionID, d.FundCode, d.TransactionDate, sg.OfferPrice
FROM Transaction d
    INNER JOIN set_gen sg ON d.TransactionID = sg.TransactionID
WHERE sg.Match_val = 1