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data dictionary. If you set autotrace on and query SELECT DUMMY FROM DUAL, you ll observe some I/O in all releases of Oracle (consistent gets). In 9i and earlier, if you query SELECT SYSDATE FROM DUAL or variable := SYSDATE in PL/SQL, you ll also see real I/O occur. However, in Oracle 10g, that SELECT SYSDATE is recognized as not needing to actually query the DUAL table (since you are not asking for the column or rowid from DUAL) and is done in a manner similar to calling a function. Therefore DUAL does not undergo a full table scan only SYSDATE is returned to the application. This small change can dramatically decrease the number of consistent gets a system that uses DUAL heavily performs.

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Using an asynchronous receive ensures no real threads are blocked for the duration of the wait. This means the previous techniques scale to many thousands of concurrent agents.

So every time we run the following query, we should be hitting the real DUAL table (since we explicitly reference the DUMMY column): sys%ORA11GR2> select tch, 2 from x$bh, (select 3 where obj = (select 4 from 5 where 6 and 7 / file#, dbablk, DUMMY dummy from dual) data_object_id dba_objects object_name = 'DUAL' data_object_id is not null)

TCH FILE# DBABLK D ---------- ---------- ---------- 1 1 929 X 2 1 928 X sys%ORA11GR2> exec dbms_lock.sleep(3.2); PL/SQL procedure successfully completed. sys%ORA11GR2> / TCH FILE# DBABLK D ---------- ---------- ---------- 2 1 929 X 3 1 928 X sys%ORA11GR2> exec dbms_lock.sleep(3.2); PL/SQL procedure successfully completed. sys%ORA11GR2> / TCH FILE# DBABLK D ---------- ---------- ---------- 3 1 929 X 4 1 928 X

Listing 13-10 shares a common structure with many of the other message-processing components you ll be looking at in this chapter, all of which are state machines. This general structure is as follows: let agent = MailboxProcessor.Start(fun inbox -> // The states of the let rec state1(args) and state2(args) ... and stateN(args) state machine = async { ... } = async { ... } = async { ... }

sys%ORA11GR2> exec dbms_locksleep(32); PL/SQL procedure successfully completed sys%ORA11GR2> / TCH FILE# DBABLK D ---------- ---------- ---------- 4 1 929 X 5 1 928 X I expect output to vary by Oracle release; you may well see more than two rows returned You might observe TCH not getting incremented every time On a multiuser system, the results will be even more unpredictable Oracle will attempt to increment the TCH once every three seconds (there is a TIM column that shows the last update time to the TCH column), but it is not considered important that the number be 100 percent accurate, as it is close Also, Oracle will intentionally "cool" blocks and decrement the TCH count over time So, if you run this query on your system, be prepared to see potentially different results.

So, in Oracle8i and above, a block buffer no longer moves to the head of the list as it used to; rather, it stays where it is in the list and has its touch count incremented Blocks will naturally tend to "move" in the list over time, however I put the word "move" in quotes because the block doesn't physically move; rather, multiple lists are maintained that point to the blocks and the block will "move" from list to list For example, modified blocks are pointed to by a dirty list (to be written to disk by DBWn) Also, as they are reused over time, when the buffer cache is effectively full, and some block with a small touch count is freed, it will be "placed" into approximately the middle of the list with the new data block.

// Enter the initial state state1(initialArgs)) That is, message-processing components typically use sets of recursive functions, each defining an asynchronous computation. Each of these functions can be thought of as a state, and one of these states is identified as the initial state. Arguments may be passed between these states just as you pass them between any other set of recursive functions.

The whole algorithm used to manage these lists is fairly complex and changes subtly from release to release of Oracle as improvements are made The actual full details are not relevant to us as developers, beyond the fact that heavily used blocks will be cached, and blocks that are not used heavily will not be cached for long..

Note If you ve been following along, running the examples in your database, take this opportunity to log out of