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

236
CHAPTER 7 ■ CONCURRENCY AND MULTI-VERSIONING
However, achieving READ COMMITTED isolation is not as cut-and-dried as it sounds. If you
look at Table 7-1, it appears straightforward. Obviously, given the earlier rules, a query executed
in any database using the READ COMMITTED isolation will behave in the same way, will
it not? It will not. If you query multiple rows in a single statement then, in almost every
other database, READ COMMITTED isolation can be as bad as a dirty read, depending on the
implementation.
In Oracle, using multi-versioning and read-consistent queries, the answer we get from
the ACCOUNTS query is the same in READ COMMITTED as it was in the READ UNCOMMITTED example.
Oracle will reconstruct the modified data as it appeared when the query began, returning the
answer that was in the database when the query started.
Let’s now take a look at how our previous example might work in READ COMMITTED mode in
other databases—you might find the answer surprising. We’ll pick up our example at the point
described in the previous table:
• We are in the middle of the table. We have read and summed the first N rows.
• The other transaction has moved $400.00 from account 123 to account 987.
• The transaction has not yet committed, so rows containing the information for
accounts 123 and 987 are locked.
We know what happens in Oracle when it gets to account 987—it will read around the
modified data, find out it should be $100.00, and complete. Table 7-4 shows how another database,
running in some default READ COMMITTED mode, might arrive at the answer.
Table 7-4. Timeline in a Non-Oracle Database Using READ COMMITTED Isolation
Time Query Account Transfer Transaction
T1 Reads row 1. Sum = $500.00 so far.
T2 Reads row 2. Sum = $740.25 so far.
T3
Updates row 1 and puts an exclusive lock
on row 1, preventing other updates and
reads. Row 1 now has $100.00.
T4 Reads row N. Sum = . . .
T5
Updates row 342,023 and puts an exclusive
lock on this row. Row 342,023 now
has $500.00.
T6 Tries to read row 342,023 and discovers
that it is locked. This session will block
and wait for this block to become
available. All processing on this
query stops.
T7
Commits transaction.
T8 Reads row 342,023, sees $500.00, and
presents a final answer that includes
the $400.00 double-counted.
The first thing to notice is that this other database, upon getting to account 987, will block
our query. This session must wait on that row until the transaction holding the exclusive lock