Is there an easy way to get the id of the row that was affected by an update statement from DBI? In this particular case, it will always be either 0 or 1 row. I didn't want the expense of having to redo the selection part of the query again to get the data, as it is kind of costly.
I am have to do the update first, because otherwise I introduce the possibility of a race-time condition between the select and the update.
You might want to read this related SO topic (I've linked to the answer by #Erwin Brandstetter) -- this is the way I've always handled it.
Depending on your database engine, you are likely to have a SELECT ... FOR UPDATE facility. You should use this to
SELECT ... FOR UPDATE the record you want to update
Save the ID from the record, and do the UPDATE using the ID instead of the original criteria
The MySQL documentation about SELECT ... FOR UPDATE may helps you, working with transactions.
Related
I use mysql trigger to update column in one of ,y DB tables called comments_count but I want to know what is best and faster??
Using mysql triggers or select query like this below:
select count(*) from comments where discussion_id=something
different types of overhead:
with the trigger you will have extra time during insert, and may get out of synch over time for some unforseen reason.
with the query, you will always get the right answer but you will need to calculate at runtime. usually, this should be very fast especially with an index on the discussion_id
I have a fairly basic query:
UPDATE the_table SET col1=[something], col2=[something else] WHERE col1 IS NULL AND col2 IS NULL LIMIT 1;
Immediately after issuing the query, the caller does:
SELECT col3 FROM the_table where col1=[something], col2=[something else];
Unfortunately, concurrent callers are claiming the same row.
I'd rather not do a SELECT FOR UPDATE, because the [select, update, select] would involve three rpcs to the database instead of two (which is bad enough.)
I gather that some dialects of sql allow UPDATE the_table WITH(UPDLOCK), but mine (galera/MySQL) does not. I find it appalling that I'd have to go through this many DB hits to execute such a basic concept. I find that most of my searching efforts end on pages that discuss dialects that DO support UPDLOCK.
Where does it go from here?
Do you have autocommit=1?
Without transactional integrity, some other connection can slip in and change the row before you execute the SELECT.
Note that there could be multiple NULL rows, so the UPDATE may be changing many rows.
Did you check the "rows affected" after the UPDATE? Maybe no rows were changed.
I think that it would be better to either execute all the queries in a transaction or to use a stored proc which will be responsible to make all the select and update stuff and then return back to you the respective data from the last select statement. Having such a flow out of transaction, raises issues as the one you describe. You need to lock the row in order not to allow other callers retrieve "dirty" (not up to date) data.
I am recently in the process of moving from oracle to mysql and would like some advice if how i am implementing something similar to sequences in mysql is a good way.
Essentially how i am currently going to implement it is by having a separate table in mysql for each sequence in oracle and have a single column which represents the last_number and increment this column when ever i insert a new row, that's one way another way i could go about doing it is by creating a single table with several rows representing each sequence and increment each row separately whenever i do an insert.
Another simpler way of doing it i could just do a select max()+1 on the relevant column when inserting data.
I'm basically thinking of switching to the select max()+1 option as it seems simpler to implement, but i would like to get some advice on what you think would be the best way of doing it out of these options, and if there is any pitfalls that i am currently not aware of when using select max()+1.
Also the reason im am not using auto_increment and the function last_insert_id() is i want to follow the ansi standard.
Thanks.
First of all: The max()+1 version is NOT guaranteed to give you a sequence, if you use transactions in a high isolation level.
The way we typically use sequences (if we can't avoid them) is to create a table with an AUTO_INCREMENT value, INSERT INTO it, SELECT last_insert_id(), DELETE FROM table WHERE field<$LASTINSERTID. This is ofcourse done in a stored procedure.
There is a read consistency problem, in that two sessions both running ...
insert into ... select max(..)+1 from ...
... at the same time both see the same value of max(...), hence they both try to insert the same new value.
You have the same problem with your table of maxima method, and you have to use a locking mechanism to avoid multiple session reading the same value. This leads to a concurrency problem where inserts to the table are serialised.
I know that triggers can be used on insert, update and delete, but what about a trigger (or sort of) on a select statement. I want to use a trigger to insert data on a table B when it is selected an existent record on a table A, it could be possible?.
Thanks in advance.
You should design your application so that database access occurs only through certain methods, and in those methods, add the monitoring you need.
Not exactly a trigger, but you can:
CREATE FUNCTION myFunc(...) BEGIN INSERT INTO myTable VALUES(...) END;
And then
SELECT myFunc(...), ... FROM otherTable WHERE id = 1;
Not an elegant solution, though.
It is not possible in the database itself.
However there are monitoring/instrumentation products for databases (e.g. for Sybase - not sure about MySQL) which track every query executed by the server, and can do anything based on that - usually store the query log into a data warehouse for later analysis, but they can just as well insert a record into table B for you, I would guess.
You can write an application which will be monitoring the query log and doing something when a select occurs. A pretty crude way to solve the problem though...
In mysql, how do I get the primary key used for an insert operation, when it is autoincrementing.
Basically, i want the new autoincremented value to be returned when the statement completes.
Thanks!
Your clarification comment says that you're interested in making sure that LAST_INSERT_ID() doesn't give the wrong result if another concurrent INSERT happens. Rest assured that it is safe to use LAST_INSERT_ID() regardless of other concurrent activity. LAST_INSERT_ID() returns only the most recent ID generated during the current session.
You can try it yourself:
Open two shell windows, run mysql
client in each and connect to
database.
Shell 1: INSERT into a table with an
AUTO_INCREMENT key.
Shell 1: SELECT LAST_INSERT_ID(),
see result.
Shell 2: INSERT into the same table.
Shell 2: SELECT LAST_INSERT_ID(),
see result different from shell 1.
Shell 1: SELECT LAST_INSERT_ID()
again, see a repeat of earlier
result.
If you think about it, this is the only way that makes sense. All databases that support auto-incrementing key mechanisms must act this way. If the result depends on a race condition with other clients possibly INSERTing concurrently, then there would be no dependable way to get the last inserted ID value in your current session.
MySQL's LAST_INSERT_ID()
The MySQL Docs describe the function: LAST_INSERT_ID()
[select max(primary_key_column_name) from table_name]
Ahhh not nessecarily. I am not an MySQL guy but there are specific way to get the last inserted id for the last completed action that are a little more robust than this. What if an insert has happened between you writing to the table and querying it? I know about because it stung me many moons ago (so yeah it does happen).
If all else fails read the manual: http://dev.mysql.com/doc/refman/5.0/en/getting-unique-id.html