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

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CHAPTER 10 ■ DATABASE TABLES 403 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. Temporary Tables 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 Chapter 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:
404 CHAPTER 10 ■ DATABASE TABLES ops$tkyte@ORA10G> create global temporary table temp_table_session 2 on commit preserve rows 3 as 4 select * from scott.emp where 1=0 5 / Table created. The ON COMMIT PRESERVE ROWS clause makes this a session-based temporary table. Rows will stay in this table until my session disconnects or I physically remove them via a DELETE or TRUNCATE. Only my session can see these rows; no other session will ever see “my” rows, even after I COMMIT. ops$tkyte@ORA10G> create global temporary table temp_table_transaction 2 on commit delete rows 3 as 4 select * from scott.emp where 1=0 5 / Table created. The ON COMMIT DELETE ROWS makes this a transaction-based temporary table. When my session commits, the rows disappear. The rows will disappear by simply giving back the temporary extents allocated to my table—there is no overhead involved in the automatic clearing of temporary tables. Now, let’s look at the differences between the two types: ops$tkyte@ORA10G> insert into temp_table_session select * from scott.emp; 14 rows created. ops$tkyte@ORA10G> insert into temp_table_transaction select * from scott.emp; 14 rows created. We’ve just put 14 rows into each TEMP table, and this shows we can “see” them: ops$tkyte@ORA10G> select session_cnt, transaction_cnt 2 from ( select count(*) session_cnt from temp_table_session ), 3 ( select count(*) transaction_cnt from temp_table_transaction ); SESSION_CNT TRANSACTION_CNT ----------- --------------- 14 14 ops$tkyte@ORA10G> commit; Since we’ve committed, we’ll see the session-based rows but not the transaction-based rows: ops$tkyte@ORA10G> select session_cnt, transaction_cnt 2 from ( select count(*) session_cnt from temp_table_session ), 3 ( select count(*) transaction_cnt from temp_table_transaction );
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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■CONTENTS v Block Buffer Cache .
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■CONTENTS vii ■CHAPTER 9 Redo a
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■CONTENTS ix ■CHAPTER 12 Dataty
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Foreword “THINK.” In 1914, Thom
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■FOREWORD xiii Tom is an aficiona
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About the Technical Reviewers ■JO
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Introduction The inspiration for th
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■INTRODUCTION xix • Exposure to
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■INTRODUCTION xxiii Next up are t
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