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

CHAPTER 10 ■ DATABASE TABLES 351
INITRANS and MAXTRANS
Each block in a segment has a block header. Part of this block header is a transaction table.
Entries will be made in the transaction table to describe which transactions have what
rows/elements on the block locked. The initial size of this transaction table is specified by the
INITRANS setting for the object. For tables, this defaults to 2 (indexes default to 2). This transaction
table will grow dynamically as needed up to MAXTRANS entries in size (given sufficient free
space on the block, that is). Each allocated transaction entry consumes 23 to 24 bytes of storage
in the block header. Note that as of Oracle 10g, MAXTRANS is ignored—all segments have a
MAXTRANS of 255.
Heap Organized Tables
A heap organized table is probably used 99 percent (or more) of the time in applications,
although that might change over time with the advent of IOTs, since they can themselves be
indexed. A heap organized table is the type of table you get by default when you issue the
CREATE TABLE statement. If you want any other type of table structure, you need to specify
that in the CREATE statement itself.
A heap is a classic data structure studied in computer science. It is basically a big area of
space, disk, or memory (disk in the case of a database table, of course), which is managed in
an apparently random fashion. Data will be placed where it fits best, rather than in any specific
sort of order. Many people expect data to come back out of a table in the same order it
was put into it, but with a heap, this is definitely not assured. In fact, rather the opposite is
guaranteed: the rows will come out in a wholly unpredictable order. This is quite easy to
demonstrate.
In this example, I will set up a table such that in my database I can fit one full row per
block (I am using an 8KB blocksize). You do not need to have the case where you only have
one row per block—I am just taking advantage of that to demonstrate a predictable sequence
of events. The following sort of behavior (that rows have no order) will be observed on tables of
all sizes, in databases with any blocksize:
ops$tkyte@ORA10GR1> create table t
2 ( a int,
3 b varchar2(4000) default rpad('*',4000,'*'),
4 c varchar2(3000) default rpad('*',3000,'*')
5 )
6 /
Table created.
ops$tkyte@ORA10GR1> insert into t (a) values ( 1);
1 row created.
ops$tkyte@ORA10GR1> insert into t (a) values ( 2);
1 row created.
ops$tkyte@ORA10GR1> insert into t (a) values ( 3);
1 row created.