IIRC it it to reduce the "overflow" of data or what oracle calls
chained rows. i.e if a table has variable length columns and 10 rows
get inserted into a datapage, if this datapage is full and one of the
variable length field gets updated the row will now "overflow" into
another datapage, but if the datapage is created with an appropriate
amount of free space the updated row will be stored in one single
datapage.
On Aug 27, 2004, at 10:27 AM, Bruce Momjian wrote:
But what is the advantage of non-full pages in Oracle?
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Adi Alurkar wrote:
Greetings,
I am not sure if this applies only to clustering but for storage in
general,
IIRC Oracle has 2 parameters that can be set at table creation :
from Oracle docs
PCTFREE integer :
Specify the percentage of space in each data block of the table,
object
table OID index, or partition reserved for future updates to the
table's rows. The value of PCTFREE must be a value from 0 to 99. A
value of 0 allows the entire block to be filled by inserts of new
rows.
The default value is 10. This value reserves 10% of each block for
updates to existing rows and allows inserts of new rows to fill a
maximum of 90% of each block.
PCTFREE has the same function in the PARTITION description and in the
statements that create and alter clusters, indexes, materialized
views,
and materialized view logs. The combination of PCTFREE and PCTUSED
determines whether new rows will be inserted into existing data blocks
or into new blocks.
PCTUSED integer
Specify the minimum percentage of used space that Oracle maintains for
each data block of the table, object table OID index, or
index-organized table overflow data segment. A block becomes a
candidate for row insertion when its used space falls below PCTUSED.
PCTUSED is specified as a positive integer from 0 to 99 and defaults
to
40.
PCTUSED has the same function in the PARTITION description and in the
statements that create and alter clusters, materialized views, and
materialized view logs.
PCTUSED is not a valid table storage characteristic for an
index-organized table (ORGANIZATION INDEX).
The sum of PCTFREE and PCTUSED must be equal to or less than 100. You
can use PCTFREE and PCTUSED together to utilize space within a table
more efficiently.
PostgreSQL could take some hints from the above.
On Aug 27, 2004, at 1:26 AM, Gaetano Mendola wrote:
Greg Stark wrote:
The discussions before talked about a mechanism to try to place new
tuples as close as possible to the proper index position.
Means this that an index shall have a "fill factor" property, similar
to
Informix one ?
From the manual:
The FILLFACTOR option takes effect only when you build an index on a
table
that contains more than 5,000 rows and uses more than 100 table
pages,
when
you create an index on a fragmented table, or when you create a
fragmented
index on a nonfragmented table.
Use the FILLFACTOR option to provide for expansion of an index at a
later
date or to create compacted indexes.
When the index is created, the database server initially fills only
that
percentage of the nodes specified with the FILLFACTOR value.
# Providing a Low Percentage Value
If you provide a low percentage value, such as 50, you allow room for
growth
in your index. The nodes of the index initially fill to a certain
percentage and
contain space for inserts. The amount of available space depends on
the
number of keys in each page as well as the percentage value.
For example, with a 50-percent FILLFACTOR value, the page would be
half
full and could accommodate doubling in size. A low percentage value
can
result in faster inserts and can be used for indexes that you expect
to grow.
# Providing a High Percentage Value
If you provide a high percentage value, such as 99, your indexes are
compacted, and any new index inserts result in splitting nodes. The
maximum density is achieved with 100 percent. With a 100-percent
FILLFACTOR value, the index has no room available for growth; any
additions to the index result in splitting the nodes.
A 99-percent FILLFACTOR value allows room for at least one insertion
per
node. A high percentage value can result in faster selects and can be
used for
indexes that you do not expect to grow or for mostly read-only
indexes.
Regards
Gaetano Mendola
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