Oracle Core - Essential Internals for DBAs, Devels. by J. Lewis

By J. Lewis

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After all, with that much extra work happening before I look at the table again, the transaction table slot I used when updating the table is sure to have been overwritten. sql, available in the Source Code/Download area of the Apress web site) even though I must have overwritten every single transaction table slot in the entire database about 50 times.

Repeat step 4 as necessary. In our case, this didn’t take many steps. 00000000 Oracle’s first step is to check (by looking at the state of slot 12 in the transaction table in undo segment header 4) that the transaction in ITL entry 0x02 is still active. Since it is, the changes it has made to this block have to be reversed out, so Oracle goes to undo block 0x018036ad, checks that its sequence (incarnation) number is 0x5ff, and then looks at record 0x3a, which tells it that “column 1 of row 0 of table 0 should be changed to value c1 02” (Oracle’s internal representation of decimal 1).

If the only thing you’ve done to show that a transaction has committed is to update a transaction table slot, then it will be up to someone else to clear up the mess you’ve left behind in the data blocks—namely, clear the lock bytes, set the Fsc/Scn to the commit SCN, and set the commit flag to C---. ■ Note There are a few places in Oracle which “hide” work by sharing it out. The principle is simple: if I tidy up the 1,000 blocks that I’ve changed, it will take me a lot of extra time; if the next 200 sessions that visit those blocks tidy up a few each, no single session will notice the overhead.

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