Says that you update 5% row each time in big table A, you still have to avoid table scan.

To avoid table scan Table A, you can create index for table...

-- parent session

IF 1 <> 0

BEGIN

-- child session 1

exec ('Select 1 as col INTO #myTemp1 ')

END

ELSE

BEGIN

-- child session 2

exec ('Select 2 as col INTO #myTemp1 ')

END

-- child session...