Why does a query slow down drastically if in the WHERE clause a constant is replaced by a parameter (having the same value)?

Question

I have a recursive query which executes very fast if the WHERE clause contains a constant but becomes very slow if I replace the constant with a parameter havin

Answer 1

For your second Query try using the OPTIMIZE FOR or OPTION(RECOMPILE) query hint to see if that forces it to recomplile based on the provided parameter value.