Query vs query - why is this one quicker than the other?

Question

I have the following two queries - the original is the first and the second is my slight "upgrade". The first takes nearly a second to run and the second finishes before I can get my finger completely off of the refresh button.

My question: Why?

The only difference between the first and second is that the first uses coalesce to get a value to compare berp.ID_PART_SESSION with and the second uses a union to put two select statements together to accomplish the same thing.

I still think the first one should be quicker (the original reason why I used coalesce) since it seems like it should be doing less work to get to the same result. Considering how weak I am deciphering execution plans, could someone please explain why the second query is so much better than the first?

declare @animator varchar
SELECT TOP 1 @animator = FULL_NAME
FROM T_BERP berp
INNER JOIN dbo.T_INTERV i ON i.ID_INTERV = berp.ID_INTERV
WHERE berp.VERSION = 1
    AND berp.PRINCIPAL = 1
    AND berp.DELETED = 0
    AND berp.CANCELLED = 0
    AND berp.ID_PART_SESSION = (
        select coalesce(pss.ID_PART_SESSION, psst.ID_PART_SESSION)
        from t_bersp b
        LEFT JOIN T_PART_SESSION pss ON b.ID_PART_SESSION = pss.ID_PART_SESSION
        LEFT JOIN T_PSS_TEMP psst ON b.ID_PSS_TEMP = psst.ID_PSS_TEMP
        where ID_BERSP = 4040)

vs

declare @animator varchar
SELECT TOP 1 @animator = FULL_NAME
FROM dbo.T_BERP berp
INNER JOIN dbo.T_INTERV i ON i.ID_INTERV = berp.ID_INTERV
WHERE berp.VERSION = 1
    AND berp.PRINCIPAL = 1
    AND berp.DELETED = 0
    AND berp.CANCELLED = 0
    AND berp.ID_PART_SESSION IN (
        select pss.ID_PART_SESSION
        from dbo.t_bersp b
        LEFT JOIN dbo.T_PART_SESSION pss ON b.ID_PART_SESSION = pss.ID_PART_SESSION
        where ID_BERSP = 4040
        union
        select psst.ID_PART_SESSION
        from dbo.t_bersp b
        LEFT JOIN dbo.T_PSS_TEMP psst ON b.ID_PSS_TEMP = psst.ID_PSS_TEMP
        where ID_BERSP = 4040)

Answer 1

I'm betting it's the coalesce statement. I believe that coalesce will end up getting applied before the where clause. So, it's actually going through each combination of the two tables, and THEN filtering on those that match the where clause.