Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 10 ■ DATABASE TABLES
■Note PCTFREE and PCTUSED are implemented differently for different table types. Some table types
employ both, whereas others only use PCTFREE, and even then only when the object is created. IOTs use
PCTFREE upon creation to set aside space in the table for future updates, but do not use PCTFREE to decide
when to stop inserting rows into a given block, for example.
The exact effect of these two parameters varies depending on whether you are using
ASSM or MSSM tablespaces. When you are using MSSM, these parameter settings control
when the block will be put on and taken off the freelist. If you are using the default values for
PCTFREE (10) and PCTUSED (40), then a block will remain on the freelist until it is 90 percent full
(10 percent free space). Once it hits 90 percent, it will be taken off the freelist and remain off
the freelist until the free space on the block exceeds 60 percent of the block.
When you are using ASSM, PCTFREE still limits if a new row may be inserted into a block,
but it does not control whether a block is on a freelist or not, as ASSM does not use freelists at
all. In ASSM, PCTUSED is simply ignored.
There are three settings for PCTFREE: too high, too low, and just about right. If you set
PCTFREE for blocks too high, you will waste space. If you set PCTFREE to 50 percent and you
never update the data, you have just wasted 50 percent of every block. On another table, however,
50 percent may be very reasonable. If the rows start out small and tend to double in size,
setting PCTFREE too small will cause row migration as you update the rows.
Row Migration
What exactly is row migration? Row migration is when a row is forced to leave the block it was
created on because it grew too large to fit on that block with the rest of the rows. I’ll illustrate a
row migration in this section. We’ll start with a block that looks like Figure 10-3.
Figure 10-3. Data block before update
Approximately one-seventh of the block is free space. However, we would like to more
than double the amount of space used by row 4 via an UPDATE (it currently consumes oneseventh
of the block). In this case, even if Oracle coalesced the space on the block as shown
in Figure 10-4, there is still insufficient room double the size of row 4 because the size of the
free space is less than the current size of row 4.