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Home -> Community -> Mailing Lists -> Oracle-L -> RE: Do fast full index scans do physical disk reads?
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
ian_at_slac.stanford.edu
-----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 <ORACLE-L_at_fatcity.com>
<ian_at_SLAC.Stan cc:
ford.EDU> Subject: RE: Do fast full index
scans do physical disk reads?
Sent by:
root_at_fatcity.c
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
ian_at_slac.stanford.edu
-----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 <ORACLE-L_at_fatcity.com>
<ian_at_SLAC.Stan cc:
ford.EDU> Subject: RE: Do fast full
index scans do physical disk reads?
Sent by:
root_at_fatcity.c
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
ian_at_slac.stanford.edu
-----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
------- ------ -------- ---------- ---------- ---------- ----------
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)
-- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: INET: Cherie_Machler_at_gelco.com Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051 San Diego, California -- Public Internet access / Mailing Lists -------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: MacGregor, Ian A. INET: ian_at_SLAC.Stanford.EDU Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051 San Diego, California -- Public Internet access / Mailing Lists -------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: INET: Cherie_Machler_at_gelco.com Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051 San Diego, California -- Public Internet access / Mailing Lists -------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: MacGregor, Ian A. INET: ian_at_SLAC.Stanford.EDU Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051 San Diego, California -- Public Internet access / Mailing Lists -------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: INET: Cherie_Machler_at_gelco.com Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051 San Diego, California -- Public Internet access / Mailing Lists -------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: MacGregor, Ian A. INET: ian_at_SLAC.Stanford.EDU Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051 San Diego, California -- Public Internet access / Mailing Lists -------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Christopher Spence INET: cspence_at_FuelSpot.com Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051 San Diego, California -- Public Internet access / Mailing Lists -------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).Received on Mon Sep 10 2001 - 12:35:43 CDT
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