Query Performance with/without PK Index

Question

Hi!
Please take a look at these queries and tell me why their
performances are
so extremely different!
1.
SELECT DISTINCT column_name #(Many Null-Values in this
Column)
FROM table_name
WHERE primary_key_index_name IN (...long list...)
AND column_name IS NOT NULL;
--> 1 Row, 120 msec
2.
#(Only Order altered:)
SELECT DISTINCT column_name
FROM table_name
WHERE column_name IS NOT NULL
AND primary_key_index_name IN (...long list...);
--> 1 Row, 2 sec (nearly 20 times slower!)
3.
#(No NOT NULL)
SELECT DISTINCT column_name
FROM table_name
WHERE primary_key_index_name IN (...long list...);
--> 1 Row, 2 sec just as No. 2!
Can anyone explain?
TIA! Dominic

Answer 1

As mentioned, you really should create explain plans for all 3
queries. I could be that the first query loaded all the block
into the buffer cache so when you ran the 2nd query, the data it
needed was already in memory.