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CHAPTER 10 DATABASE TABLES
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15 organization index compress 1 ) 16 RETURN AS VALUE 17 / Table created. we now get the following set of objects. Instead of having a conventional table EMP_NT, we have an IOT EMPS_NT as signified by the index structure overlaid on the table in Figure 10-12.
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Figure 10-12. Nested table implemented as an IOT Where the EMPS_NT is an IOT using compression, it should take less storage than the original default nested table and it has the index we badly need.
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I do not use nested tables as a permanent storage mechanism myself, for the following reasons: The unnecessary storage overhead of the RAW(16) columns that are added. Both the parent and child table will have this extra column. The parent table will have an extra 16-byte RAW for each nested table column it has. Since the parent table typically already has a primary key (DEPTNO in my examples), it makes sense to use this key in the child tables, not a system-generated key. The unnecessary overhead of the additional unique constraint on the parent table, when it typically already has a unique constraint. The nested table is not easily used by itself, without using unsupported constructs (NESTED_TABLE_GET_REFS). It can be un-nested for queries, but not mass updates. I have yet to find a table in real life that isn t queried by itself.
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CHAPTER 10 DATABASE TABLES
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I do use nested tables heavily as a programming construct and in views. This is where I believe they are in their element. As a storage mechanism, I much prefer creating the parent/ child tables myself. After creating the parent/child tables, we can, in fact, create a view that makes it appear as if we had a real nested table. That is, we can achieve all of the advantages of the nested table construct without incurring the overhead. If you do use a nested table as a storage mechanism, be sure to make it an IOT to avoid the overhead of an index on the NESTED_TABLE_ID and the nested table itself. See the previous section on IOTs for advice on setting them up with overflow segments and other options. If you do not use an IOT, make sure to create an index on the NESTED_TABLE_ID column in the nested table to avoid full scanning it to find the child rows.
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Temporary tables are used to hold intermediate resultsets, for the duration of either a transaction or a session. The data held in a temporary table is only ever visible to the current session no other session will see any other session s data, even if the current session COMMITs the data. Multiuser concurrency is not an issue with regard to temporary tables either, as one session can never block another session by using a temporary table. Even if we lock the temporary table, it will not prevent other sessions using their temporary table. As we observed in 9, temporary tables generate significantly less redo than regular tables. However, since they must generate undo information for the data they contain, they will generate some amount of redo. UPDATEs and DELETEs will generate the largest amount; INSERTs and SELECTs the least amount. Temporary tables will allocate storage from the currently logged-in user s temporary tablespace, or if they are accessed from a definer rights procedure, the temporary tablespace of the owner of that procedure will be used. A global temporary table is really just a template for the table itself. The act of creating a temporary table involves no storage allocation; no INITIAL extent is allocated, as it would be for a regular table. Rather, at runtime when a session first puts data into the temporary table, a temporary segment for that session will be created. Since each session gets its own temporary segment (not just an extent of an existing segment), every user might be allocating space for her temporary table in different tablespaces. USER1 might have his temporary tablespace set to TEMP1, so his temporary tables will be allocated from this space. USER2 might have TEMP2 as her temporary tablespace, and her temporary tables will be allocated there. Oracle s temporary tables are similar to temporary tables in other relational databases, with the main exception being that they are statically defined. You create them once per database, not once per stored procedure in the database. They always exist they will be in the data dictionary as objects, but they will always appear empty until your session puts data into them. The fact that they are statically defined allows you to create views that reference temporary tables, to create stored procedures that use static SQL to reference them, and so on. Temporary tables may be session based (data survives in the table across commits but not a disconnect/reconnect). They may also be transaction based (data disappears after a commit). Here is an example showing the behavior of both. I used the SCOTT.EMP table as a template:
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