Gibson wrote:
I seem to get this error "user postgres could not be created, user
account already exists" when trying to install postgres. Anyone knows
how to fix this?
Sounds to me like the user postgres on the OS already exists.
Joshua D. Drake
Erik Jones wrote:
Erik Jones wrote:
Ok, consider the following table definition:
CREATE TABLE stats (
id SERIAL PRIMARY KEY,
hits bigint default 0,
clickthrus bigint default 0,
referrals bigint default 0);
Now, this table has a lot of rows that are constantly being updated
by queries of the following form:
UPDATE stats
SET clickthrus = clickthrus + #
WHERE id = blah; -- sub various values for # and blah
There can be, and often are, multiple updates for the same row
coming in at the same time, but never for the same field. My
understanding of the locking involved is that updates take out
row-exclusive locks to prevent other transactions from modifying the
data and to serialize with other updates. So, multiple update
statements to the same row come in, the first to arrive is granted a
row-exclusive lock and the other wait. When the first is finished
and commits, the second to have arrived get the lock, and so forth.
Here is what I am seeing all through my logs:
2006-08-29 03:17:25 CDT 16074 192.168.1.173(35190):STATEMENT: ABORT
2006-08-29 03:17:25 CDT 8553 192.168.1.168(42707):ERROR: deadlock
detected
2006-08-29 03:17:25 CDT 8553 192.168.1.168(42707):DETAIL: Process
8553 waits for ShareLock on transaction 1548224183; blocked by
process 5499.
Process 5499 waits for ShareLock on transaction 1548224182; blocked
by process 8553.
2006-08-29 03:17:25 CDT 8553 192.168.1.168(42707):STATEMENT: UPDATE
stats
SET hits = hits + 3
WHERE id = 271524;
or,
2006-08-29 08:47:31 CDT 12479 192.168.1.168(46125):ERROR: deadlock
detected
2006-08-29 08:47:31 CDT 12479 192.168.1.168(46125):DETAIL: Process
12479 waits for ExclusiveLock on tuple (3024,45) of relation 33942 of
database 33325; blocked by process 12513
.
Process 12513 waits for ShareLock on transaction 1550567046; blocked
by process 12495.
Process 12495 waits for ShareLock on transaction 1550569729; blocked
by process 12479.
2006-08-29 08:47:31 CDT 12479 192.168.1.168(46125):STATEMENT: UPDATE
stats
SET click_thrus = clickthrus + 1
WHERE id = 275359;
What's with ShareLock s on transactions? Where do those come from?
I should also note that each of those updates occurs in it's own
transactions, but that they do not attempt to modify any other rows in
that table before commiting. They do, however, delete rows in
another common table (where they pulled the stat counts from), but the
rows they delete are disjunct.
The whole process/algorithm is such:
1. Get rows matching X from temp table.
2. Accumulate values from X and update row and field corresponding to
X in stats table.
3. Delete rows collected in step one.
4. Commit.
5. Repeat from step 1.
With multiple processes using the same algo and tables but for
different values of X.
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