Re: IOT, memory and transaction time

On Apr 17, 10:39 am, Robert Klemme <shortcut...@xxxxxxxxxxxxxx> wrote:
On 17.04.2007 11:26, d...@xxxxxxxxxxxx wrote:

We have a table with 4 columns - col1 int, col2 int, col3 char(32),
col4 char(32).

Previously there were 2 indexes
unique index1 on col3, col1
index2 on col2, col1

Reads always read all columns.

We have recently changed the way this table works. The columns are all
the same but because of the way we put data in we have changed the
indexes.

Please provide complete DDL for old and new table layout. You can
easily generate it with DBMS_METADATA.GET_DDL if your verison of Oracle
includes it.

Now the table is an Index Ordered table with col2,col3 as PRIMARY KEY.

IOT = Index Organized Table.

There is an additional index on col3,col2.

Why do you put another index on the same set of columns?



Reads have improved quite a lot but there is a problem with the
writes. Previously, when adding 40,000 records at a time, write times
were pretty consistent no matter how big the table (this transaction
used to actually involve 200 updates of the same table followed by
40,000 inserts). With the new indexes, they get progressively worse
with every write plateauing at around 10 minutes!! (this new
transaction involves updating a dummy table with one record and one
column 200 times (using a where clause that references the other
table), followed by 40,000 inserts). The update part of the
transaction basically checks that no-one else has updated since you
read.

Why have write times changed so dramatically? Why do they get
progressively worse? If it was the update statement that references
the other table with a where clause, then why have Read times improved
(using pretty much the same clause)? So I figured it probably wasn't
the Update statement. So, looking at the Insert, why has this become
so different. The columns are the same, it is just that one is Index
Ordered and one isn't. The 40000 inserts will all have a col2 value
that is the same but is greater than any previous write of col2. The
col3 values may not be in order when written back - so is it just a
memory issue? If we allocate more memory to the Oracle instance will
that solve it? Does the memory have to be allocated to a specific area
or is the SGA sufficient?

My first guess would be that because of the changed physical structure
inserts are generally more complex and generate more IO.

Kind regards

robert

Thanks Robert,

Here is the DDL...

New DDL:
CREATE TABLE R_TABLE1 (
deleted INTEGER NOT NULL,
versionNo INTEGER NOT NULL,
Id char(32) NOT NULL,
Rel char(32) NOT NULL,
CONSTRAINT R_TABLE1_1 PRIMARY KEY(versionNo,Id, Rel),
FOREIGN KEY (deleted) REFERENCES T_TRANSACTIONS (TransNum),
FOREIGN KEY (versionNo) REFERENCES T_TRANSACTIONS (TransNum),
CHECK (versionNo >= deleted))
ORGANIZATION INDEX;

CREATE INDEX R_TABLE1_2 ON R_TABLE1 (Id, versionNo);


Old DDL:
CREATE TABLE R_TABLE1(
deleted INTEGER NOT NULL,
versionNo INTEGER NOT NULL,
Id char(32) NOT NULL,
Rel char(32) NOT NULL,
FOREIGN KEY (deleted) REFERENCES T_TRANSACTIONS (TransNum),
FOREIGN KEY (versionNo) REFERENCES T_TRANSACTIONS (TransNum),
CHECK (versionNo > deleted));

CREATE UNIQUE INDEX R_TABLE1_1 ON R_TABLE1 (Id, deleted, Rel);
CREATE INDEX R_TABLE1_2 ON R_TABLE1 (versionNo, deleted);

There are 2 basic reads on this table.
1) the latest version of a record
2) all the changes since the last read version

In the old system this meant
1) select * from r_table1 where deleted = -1 (because every time a
change was made, it would update the deleted column of the existing
version = the current transNum and then insert a new version with
deleted = -1)
AND
select * from r_table1 where deleted between 0 and current transNum
(just in case someone made changes between you starting your read and
ending it - consistent read of numerous tables)

2) select * from r_table1 where versionNo > @lastRead and versionNo <=
@currentTransNum
AND
select * from r_table1 where versionNo > currentTransNum and deleted >
@lastRead and deleted <= currentTrans (again for consistent read)

In the new system it just involves one read for each because no update
takes place, a new version just supercedes the old version by virtue
of a bigger versionNo:
1) select * from r_table1 t1 where versionNo = (select max(versionNo)
from r_table1 t2 where t1.id = t2.id and t2.versionNo <=
@currentTransNum) and deleted = -1
2) select * from r_table1 t1 where versionNo = (select max(versionNo)
from r_table1 t2 where t1.id = t2.id and t2.versionNo > @lastRead and
t2.versionNo <= @currentTransNum)

In the old system the write transaction used to do:
1) Update R_TABLE1 Set deleted = versionNo, versionNo =
@currentTransNum where id = @id and deleted = -1 and versionNo =
@versionNo
ONCE FOR EACH value of Id
(If 1 record updated then no-one has updated since our read)
2) Insert into R_TABLE1(deleted, versionNo, Id, Rel) Values (-1,
@currentTransNum, @id, @rel)
for as many relationship tuples as required.

In the new system the write transaction does this:
1) Updates DUMMY Set Col1 = 1 WHERE @versionNo = (SELECT
MAX(versionNo) from R_TABLE1 where id = @id and versionNo <
@currentTransNum)
(If 1 record updated then no-one has updated since our read)
2) Same insert as old one above.


As far as secondary index is concerned, the application goes out in an
Oracle and a SQL Server flavour. In SQL Server this additional index
seemed to improve performance being used in some of the subselects.

The problem is, that the new version of our schema does not perform
like this under SQL Server, only under Oracle. Obviously in SQL Server
we use a CLUSTERED INDEX but as far as I am aware that is just the
same as an IOT.

Thanks for any advice
Phil


.