Declare table and join it shows slow, but fast when using real number

Question

I've try following SQL and it was slow when I useing declare @NewCityJudge table and join it, but it was fast when I convert table into real number and join it.

-- input id into @NewCityJudge, only one record
declare @NewCityJudge table(CountryId int)
insert into @NewCityJudge 
select CountryId from .... 


SELECT TOP (300) *
MyTable as b
    join ComponentLanguageIndex as c on c.id = b.[key]
    join ComponentCountryTags e on c.ComponentId = e.ComponentId
    join @NewCityJudge  as d on d.CountryId = e.CountryId -- join @NewCityJudge here

But it faster when using

SELECT TOP (300) *
MyTable  as b
    join ComponentLanguageIndex as c on c.id = b.[key]
    join ComponentCountryTags e on c.ComponentId = e.ComponentId
where CountryId in (39)

The @NewCityJudge always less 5 records.

The first way takes 5 seconds, The second way takes 500 ms.

Thanks

PS. It was fast when using #NewCityJudge temp table, but I afraid it cause some transaction issue

Answer 1

As opposed to a Join, you could use the following:

SELECT TOP (300) *
MyTable  as b
    join ComponentLanguageIndex as c on c.id = b.[key]
    join ComponentCountryTags e on c.ComponentId = e.ComponentId
where e.CountryId IN (Select CountryId from @NewCityJudge)

Anytime you use TOP (#) on a table with some joins, it is less than ideal. I would also like to understand why TOP (300) ? Is this for testing purposes ? How many records are in MyTable ? If you remove TOP (300), you may find your query time issue resolved.

Answer 2

I solved this problem by using temp table instead of parameter.

Create table  #NewCityJudge(CountryId int) insert into #NewCityJudge  select CountryId from .... 


SELECT TOP (300) * MyTable as b
    join ComponentLanguageIndex as c on c.id = b.[key]
    join ComponentCountryTags e on c.ComponentId = e.ComponentId
    join #NewCityJudge  as d on d.CountryId = e.CountryId