Why are the performances of these 2 queries so different?

Question

I have a stored proc that searches for products (250,000 rows) using a full text index.

The stored proc takes a parameter that is the full text search condition. This pa

Answer 1

The first query plan looks straightforward:

1. a full text search to resolve CONTAINS(Name, @Filter)
2. an index scan to look up the other columns of the matched rows
3. combine the two using a hash join

The concatenation operator forms a union of two recordsets. So it looks like the second query is doing:

1. an index scan (later used to look up other columns)
2. a constant scan. I assume it's treating your query as not parameterized, so the query plan doesn't have to work for any other value of @Filter. If correct, the constant scan resolves @Filter is not null.
3. a full text search to resolve CONTAINS(Name, @Filter)
4. unions the result of 3 with the empty set from 2
5. loop joins the result of 1 and 4 to look up the other columns

A hash join trades memory for speed; if your system has enough memory, it's much faster than a loop join. This can easily explan a 10-100x slowdown.

One fix is to use two distinct queries:

if @Filter is null
    SELECT TOP 100 ID FROM dbo.Products
else
    SELECT TOP 100 ID FROM dbo.Products WHERE CONTAINS(Name, @Filter)