Tom,
Well you got me sort of. I ran a few quick tests on a table with 500K
rows. The return times were almost always identical. The main difference
between a Pk with a number and a char/varchar is storage. a 40 digit
number takes 4bytes of space. A 40 character string takes 10 bytes.
This translated into the character datatype moving 30 more bytes per
query over sql*net than the number datatype. On a heavily used
application hitting a backend Oracle DB via sql*net, those extra bytes
could make a difference in response time. but I guess that is more of a
network bottleneck than a database one.
here are my results:
char(40) Pk:
Elapsed: 00:00:00.52
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=10)
1 0 INDEX (UNIQUE SCAN) OF 'PKTEST_CHAR_PK' (UNIQUE) (Cost=2 C
ard=1 Bytes=10)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
239 bytes sent via SQL*Net to client
253 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
number datatype:
Elapsed: 00:00:00.51
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=4)
1 0 INDEX (UNIQUE SCAN) OF 'PKTEST_PK' (UNIQUE) (Cost=2 Card=1
Bytes=4)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
218 bytes sent via SQL*Net to client
246 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
-Brian
On Wed, 2002-04-17 at 11:52, Mercadante, Thomas F wrote:
> All,
>
> Does anyone have any specific metrics demonstrating that a PK that is based
> on a number field is faster than a PK based on a character field?
>
> I've seen it mentioned a couple of times today under the "Design Question"
> topic.
>
> It doesn't make any sense to me that one or the other would be faster.
> After all, we are talking about comparison searches within the B-Tree index
> structure. Why searching down the tree for a number is any faster than a
> char is lost on me.
>
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Author: Brian Haas
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