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CHAPTER 10 DATABASE TABLES
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Figure 10-1. HWM depiction In an MSSM tablespace, segments have a definite HWM. In an ASSM tablespace, however, there is an HWM and a low HWM (see Figure 10-2). In MSSM, when the HWM is advanced (e.g., as rows are inserted), all of the blocks are formatted and valid, and Oracle can read them safely. With ASSM, however, when the HWM is advanced, Oracle doesn t format all of the blocks immediately they are only formatted and made safe to read upon their first use. So, when full scanning a segment, we have to know if the blocks to be read are safe or unformatted (meaning they contain nothing of interest and we do not process them). To make it so that not every block in the table need go through this safe/not safe check, Oracle maintains a low HWM and an HWM. Oracle will full scan the table up to the HWM and for all of the blocks below the low HWM, it will just read and process them. For blocks between the low HWM and the HWM, it must be more careful and refer to the ASSM bitmap information used to manage these blocks to see which of them it should read and which it should just ignore.
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Figure 10-2. Low HWM depiction
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CHAPTER 10 DATABASE TABLES
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When you use an MSSM tablespace, the freelist is where Oracle keeps tracks of blocks under the HWM for objects that have free space on them.
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Note Freelists and freelist groups do not pertain to ASSM tablespaces at all; only MSSM tablespaces use
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Each object will have at least one freelist associated with it, and as blocks are used, they will be placed on or taken off of the freelist as needed. It is important to note that only blocks under the HWM of an object will be found on the freelist. The blocks that remain above the HWM will be used only when the freelists are empty, at which point Oracle advances the HWM and adds these blocks to the freelist. In this fashion, Oracle postpones increasing the HWM for an object until it has to. An object may have more than one freelist. If you anticipate heavy INSERT or UPDATE activity on an object by many concurrent users, then configuring more than one freelist can have a major positive impact on performance (at the cost of possible additional storage). Having sufficient freelists for your needs is crucial. FREELISTS can be a huge positive performance influence (or inhibitor) in an environment with many concurrent inserts and updates. An extremely simple test can show the benefits of setting FREELISTS correctly. Consider this relatively simple table: ops$tkyte@ORA10GR1> create table t ( x int, y char(50) ) tablespace MSSM; Table created. Using five concurrent sessions, we start inserting into this table like wild. If we measure the systemwide wait events for block-related waits both before and after inserting, we will find large waits, especially on data blocks (trying to insert data). This is frequently caused by insufficient freelists on tables (and on indexes, but we ll cover that in detail in the next chapter). I used Statspack for this I took a statspack.snap, executed a script that started the five concurrent SQL*Plus sessions, and waited for them to exit before taking another statspack.snap. The script these sessions ran was simply as follows: begin for i in 1 .. 100000 loop insert into t values ( i, 'x' ); end loop; commit; end; / exit; Now, this is a very simple block of code, and I m the only user in the database here. I should get the best possible performance. I have plenty of buffer cache configured, my redo
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