Yes it's fun getting Chris, our self-touting genuis. However, the point is not that
unique indexes cannot be compressed, but that the prefix length canot be mre that one
less than the numbewr of coluymns in the index.
Ian A MacGregor
Stanford Linear Accelerator Center
[EMAIL PROTECTED]
-----Original Message-----
Sent: Monday, September 10, 2001 2:55 PM
To: Multiple recipients of list ORACLE-L
Whoa Chris, gotcha on that one. :)
The combination of columns may be unique, but you can certainly compress
a unique index for significant space savings.
The unique index I used in testing had three columns, ordered most
selective
first. By reversing the column order of the index and compressing it,
the index went from 180 Megs to 60 megs.
The cardinality of the new leading column was 26 in table of 1.5 million
rows.
Jared
Christopher
Spence To: Multiple recipients of list ORACLE-L
<[EMAIL PROTECTED]>
<cspence@FuelS cc:
pot.com> Subject: RE: Do fast full index scans do
physical disk reads?
Sent by:
[EMAIL PROTECTED]
om
09/10/01 01:20
PM
Please respond
to ORACLE-L
Yes, there is no point in compressing all columns of a unique index as it
would result in 0% compression as they are unique.
"Do not criticize someone until you walked a mile in their shoes, that way
when you criticize them, you are a mile a way and have their shoes."
Christopher R. Spence
Oracle DBA
Phone: (978) 322-5744
Fax: (707) 885-2275
Fuelspot
73 Princeton Street
North, Chelmsford 01863
-----Original Message-----
Sent: Monday, September 10, 2001 3:55 PM
To: Multiple recipients of list ORACLE-L
The restriction is on unique indexes.
1* select column_name from dba_ind_COLUMNS where INDEX_NAME = 'WOLO_PK'
SQL> /
COLUMN_NAME
----------------------------------------------------------------------------
----
PERSON_ID
INSTITUTION_CODE
ALTER INDEX CASEPUPPY.WOLO_PK
REBUILD COMPRESS 2 TABLESPACE PEPII_INDEX
/
REBUILD COMPRESS 2 TABLESPACE PEPII_INDEX
*
ERROR at line 2:
ORA-25194: invalid COMPRESS prefix length value
ALTER INDEX CASEPUPPY.WOLO_PK
REBUILD COMPRESS 1 TABLESPACE PEPII_INDEX
/
Index altered.
----------------------------------------------------------------------------
----------------------------------------
This was done on an 8.1.6.3 database. As far as what's done in practice, I
would hazard accepting the Oracle defaults for prefix length values is the
most common. The documentation states:
"For unique indexes, the valid range of prefix length values is from 1 to
the number of key columns minus 1. The default prefix length is the number
of key columns minus 1.
For nonunique indexes, the valid range of
prefix length values is from 1 to the number of key columns. The default
prefix length is the number of
key columns."
Ian MacGregor
Stanford Linear Accelerator Center
[EMAIL PROTECTED]
-----Original Message-----
Sent: Monday, September 10, 2001 11:26 AM
To: Multiple recipients of list ORACLE-L
Actually you can create compressed indexes upto the size of the columns. In
other words, the last column in a concentated index can be compressed.
Although most practice does not.
"Do not criticize someone until you walked a mile in their shoes, that way
when you criticize them, you are a mile a way and have their shoes."
Christopher R. Spence
Oracle DBA
Phone: (978) 322-5744
Fax: (707) 885-2275
Fuelspot
73 Princeton Street
North, Chelmsford 01863
-----Original Message-----
Sent: Monday, September 10, 2001 1:45 PM
To: Multiple recipients of list ORACLE-L
You cannot compress single column unique indexes. The rule is: you can
compress up to n-1 columns of a unique index where n = the number of
columns
in the index. A multi-column compressed index should, for maximum effect,
have as its leading column the one with greatest number of repeated
values.
This is in conflict with the rule that states to put the column with the
highest cardinality first.
Bear in mind compressing an index is not cost free. The CPU will need to
do
more work to read the index; however the cost of the work will be less than
doing a physical I/O.
You can compress all columns of a non-unique index.
Ian MacGregor
Stanford Linear Accelerator Center
[EMAIL PROTECTED]
-----Original Message-----
Sent: Monday, September 10, 2001 5:50 AM
To: Multiple recipients of list ORACLE-L
Ian,
I'll look at compressing the index. Does that only work on unique indexes
or can you
do it on non-unique multi-column indexes as well?
Thanks,
Cherie
"MacGregor,
Ian A." To: Multiple recipients of
list
ORACLE-L <[EMAIL PROTECTED]>
<[EMAIL PROTECTED] cc:
ford.EDU> Subject: RE: Do fast full
index
scans do physical disk reads?
Sent by:
[EMAIL PROTECTED]
om
09/07/01 03:26
PM
Please respond
to ORACLE-L
The advantage of the fast full index scan is that it should read fewer
blocks than the full table scan. Index compression may help reduce the
number of blocks read even further. A unique index mist be at least two
columns wide to benefit from compression.
Ian MacGregor
Stanford Linear Accelerator Center
[EMAIL PROTECTED]
-----Original Message-----
Sent: Friday, September 07, 2001 5:20 AM
To: Multiple recipients of list ORACLE-L
Ian,
The last one I looked at it was cached, I guess. I could purposely cache
the
table (and index) if it was small, though.
I'm confused though. Isn't the whole benefit of the fast, full index scan
that you
don't have to go against the table, thereby avoiding those physical reads?
Or, in the case where the index isn't cached, is the benefit that you don't
have to read all of the columns in the table that aren't part of the index?
Thanks for your reply,
Cherie
"MacGregor,
Ian A." To: Multiple recipients of
list ORACLE-L <[EMAIL PROTECTED]>
<[EMAIL PROTECTED] cc:
ford.EDU> Subject: RE: Do fast full
index scans do physical disk reads?
Sent by:
[EMAIL PROTECTED]
om
09/07/01 01:05
AM
Please respond
to ORACLE-L
There is no rule that says an index will be cache. Yes physical reads are
being done. If the unique index is composed of more than one column look
into compressing it.
Ian MacGregor
Stanford Linear Accelerator Center
[EMAIL PROTECTED]
-----Original Message-----
Sent: Thursday, September 06, 2001 1:51 PM
To: Multiple recipients of list ORACLE-L
I am confused by the output from tkprof below. An fast full index
scan is being performed. However, from the statistics, it looks as
thought 649 physical disk reads are being performed. Is that actually
the case? Are physical disk reads being done?
Thanks,
Cherie Machler
Oracle DBA
Gelco Information Network
****************************************************************************
****
Select SD.KS_OBJECTID as CONCEPTID
>From kbowner.KS_SHORTDESCRIPTION SD
Where SD.KS_DESCRIPTIONTYPE = 'CPTNAME' And
UPPER(SD.KS_DESCRIPTIONTEXT) = ''
call count cpu elapsed disk query current
rows
------- ------ -------- ---------- ---------- ---------- ----------
----------
Parse 1 0.03 0.03 0 0 0
0
Execute 1 0.00 0.00 0 0 0
0
Fetch 1 0.30 0.30 649 649 4
0
------- ------ -------- ---------- ---------- ---------- ----------
----------
total 3 0.33 0.33 649 649 4
0
Rows Row Source Operation
------- ---------------------------------------------------
0 INDEX FAST FULL SCAN (object id 5286)
Rows Execution Plan
------- ---------------------------------------------------
0 SELECT STATEMENT GOAL: CHOOSE
0 INDEX GOAL: ANALYZED (FAST FULL SCAN) OF 'SYS_C001069' (UNIQUE)
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