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CHAPTER 10 DATABASE TABLES
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Hash clustered tables: These tables are similar to clustered tables, but instead of using a B*Tree index to locate the data by cluster key, the hash cluster hashes the key to the cluster to arrive at the database block the data should be on. In a hash cluster, the data is the index (metaphorically speaking). These tables are appropriate for data that is read frequently via an equality comparison on the key. Sorted hash clustered tables: This table type is new in Oracle 10g and combines some aspects of a hash clustered table with those of an IOT. The concept is as follows: you have some key value that rows will be hashed by (say, CUSTOMER_ID), and then a series of records related to that key that arrive in sorted order (timestamp-based records) and are processed in that sorted order. For example, a customer places orders in your order entry system, and these orders are retrieved and processed in a first in, first out (FIFO) manner. In such as system, a sorted hash cluster may be the right data structure for you. Nested tables: These are part of the object-relational extensions to Oracle. They are simply system-generated and -maintained child tables in a parent/child relationship. They work in much the same way as EMP and DEPT in the SCOTT schema. EMP is considered to be a child of the DEPT table, since the EMP table has a foreign key, DEPTNO, that points to DEPT. The main difference is that they are not stand-alone tables like EMP. Temporary tables: These tables store scratch data for the life of a transaction or the life of a session. These tables allocate temporary extents, as needed, from the current user s temporary tablespace. Each session will see only the extents that session allocates; it will never see any of the data created in any other session. Object tables: These tables are created based on an object type. They have special attributes not associated with non-object tables, such as a system-generated REF (object identifier) for each row. Object tables are really special cases of heap, index organized, and temporary tables, and they may include nested tables as part of their structure as well. External tables: These tables are not stored in the database itself; rather, they reside outside of the database in ordinary operating system files. External tables in Oracle9i and above give you the ability to query a file residing outside the database as if it were a normal table inside the database. They are most useful as a means of getting data into the database (they are a very powerful data-loading tool). Furthermore, in Oracle 10g, which introduces an external table unload capability, they provide an easy way to move data between Oracle databases without using database links. We will look at external tables in some detail in 15. Here is some general information about tables, regardless of their type: A table can have up to 1,000 columns, although I recommend against a design that does contain the maximum number of columns, unless there is some pressing need. Tables are most efficient with far fewer than 1,000 columns. Oracle will internally store a row with more than 254 columns in separate row pieces that point to each other and must be reassembled to produce the entire row image.
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CHAPTER 10 DATABASE TABLES
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A table can have a virtually unlimited number of rows, although you will hit other limits that prevent this from happening. For example, typically a tablespace can have at most 1,022 files (although there are new BIGFILE tablespaces in Oracle 10g that will get you beyond these file size limits, too). Say you have 32GB files that is to say, 32,704GB per tablespace. This would be 2,143,289,344 blocks, each of which is 16KB in size. You might be able to fit 160 rows of between 80 to 100 bytes per block. This would give you 342,926,295,040 rows. If you partition the table, though, you can easily multiply this number many times. For example, consider a table with 1,024 hash partitions that would be 1024 342,926,295,040 rows. There are limits, but you ll hit other practical limitations before even coming close to these figures. A table can have as many indexes as there are permutations of columns (and permutations of functions on those columns). With the advent of function-based indexes, the true number of indexes you could create theoretically becomes infinite! Once again, however, practical restrictions will limit the actual number of indexes you will create and maintain. There is no limit to the number of tables you may have, even within a single database. Yet again, practical limits will keep this number within reasonable bounds. You will not have millions of tables (this many is impracticable to create and manage), but you may have thousands of tables. In the next section, we ll look at some of the parameters and terminology relevant to tables. After that, we ll jump into a discussion of the basic heap-organized table, and then move on to examine the other types.
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