Check for existence of a record

Question

I have a table XX_TEMP. Let us say the columns are inventory_item, organization_id and description
The values are:
Inventory_item
Organization
Description
200
m1
Sample
200
m2
Not Sample
400
m4
check
700
m5
Test
I just want to check the existence of an item in the table, I have written two queries and would like to know which one is better in terms of performance:
Q1:
select count(1) from xx_temp where inventory_item=200 and rownum=1;
Q2:
select count(1) from dual where exists (select 1 from xx_temp where inventory_item=200);
Both Q1 and Q2 return the same result. In fact, I was surprised with the result from Q1 as I expected that the rownum would be evaluated after the where condition. I expected Q1 to return 2
I thought that the below query:
select count(1) from xx_temp where inventory_item=200;
and Q1 would return the same result as rownum would be evaluated at end. In effect, I've 2 questions:
1. Isn't rownum calculated at the end?
2. What is the best way in terms of performance to check for an existence of record?

Answer 1

Internally this is how it works:
select count(*) from xx_temp where inventory_item=200 and rownum=1;
  COUNT(*)
         1
1 row selected.
Execution Plan
   0       SELECT STATEMENT Optimizer Mode=ALL_ROWS (Cost=4 Card=1 Bytes=13)
   1    0    SORT AGGREGATE (Card=1 Bytes=13)
   2    1      COUNT STOPKEY
   3    2        TABLE ACCESS FULL XX_TEMP (Cost=4 Card=2 Bytes=26)
Statistics
          5  user calls
          0  physical read total multi block requests
          0  physical read total bytes
          0  cell physical IO interconnect bytes
          0  commit cleanout failures: block lost
          0  IMU commits
          0  IMU Flushes
          0  IMU contention
          0  IMU bind flushes
          0  IMU mbu flush
          1  rows processed
Plan
1 Every row in the table XX_TEMP  is read.
2 Processing was stopped when the specified number of rows from step 1 were processed.
3 The rows were sorted to support a group operation (MAX,MIN,AVERAGE, SUM, etc).
4 Rows were returned by the SELECT statement.
COUNT STOPKEY knows how many rows you want and will just keep calling its child function under it in the execution plan tree to get more and more rows, until the required amount of rows have been returned. Here it stopped at 1 iteration.
And to answer your second question : as to which is the fastest way to to check  for an existence of record :
Answer would be it depends on your requirement.  possible answers are :  Rowid --  fastest way to check for a row.   similar answers can be Index etc.. but all this is relative to what you work with.
Cheers,
Manik.