I'm trying to make an experimental web application which minimises redundant data.
I have three example tables set up like so:
Table one
ID | created_at (unix timestamp) | updated_at (unix timestamp)
Table two
ID | Foreign Key to table one | Title
Table three (pages)
ID | Foreign Keys to both table one and two | Content | Metadata
The idea being that everything created in the application will have a creation/edit time.
Many (but not all) things will have a title (For example a page or a section for a page to go into).
Finally, some things will have attributes specific to themselves, eg content and metadata for a page.
I'm trying to work out the best way to enter data into multiple tables. I know I could do multiple insert queries from PHP, keep track of rows created in the current transaction and delete those rows should a later part of the transaction fail. However, if the PHP script dies completely, it may stop before all of the deletions can be completed.
Does MySQL have any inbuilt logic which would allow the insert query to be split up? Would a trigger be able to handle this type of transaction or is it beyond its capabilities?
Any advice, thoughts or ideas would be greatly appreciated.
Thanks!
A solution would be to use Transactions, which allow to get "all or nothing" behaviour.
The idea is the following :
you start a transaction
you do your inserts/updates
if everything is OK, you commit the transaction ; which will save everything you did during this transaction
if not, you rollback the transaction ; and everything you did in it will be cancelled.
if you don't commit and disconnected (if your PHP script dies, for instance), nothing will be commited, and what you did during the un-commited transaction will automatically be rolled-back.
For more informations, you can take a look at 12.4.1. START TRANSACTION, COMMIT, and ROLLBACK Syntax, for MySQL.
Note that transactions are only available for some DB engines :
MyISAM doesn't support transactions
InnoDB does (it also supports foreign keys, for instance -- it's far more advanced that MyISAM).
For multiple inserts you can create a procedure and on PHP you call the procedure.
Related
I have got a table which has an id (primary key with auto increment), uid (key refering to users id for example) and something else which for my question won’t matter.
I want to make, lets call it, different auto-increment keys on id for each uid entry.
So, I will add an entry with uid 10, and the id field for this entry will have a 1 because there were no previous entries with a value of 10 in uid. I will add a new one with uid 4 and its id will be 3 because I there were already two entried with uid 4.
...Very obvious explanation, but I am trying to be as explainative an clear as I can to demonstrate the idea... clearly.
What SQL engine can provide such a functionality natively? (non Microsoft/Oracle based)
If there is none, how could I best replicate it? Triggers perhaps?
Does this functionality have a more suitable name?
In case you know about a non SQL database engine providing such a functioality, name it anyway, I am curious.
Thanks.
MySQL's MyISAM engine can do this. See their manual, in section Using AUTO_INCREMENT:
For MyISAM tables you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column) + 1 WHERE prefix=given-prefix. This is useful when you want to put data into ordered groups.
The docs go on after that paragraph, showing an example.
The InnoDB engine in MySQL does not support this feature, which is unfortunate because it's better to use InnoDB in almost all cases.
You can't emulate this behavior using triggers (or any SQL statements limited to transaction scope) without locking tables on INSERT. Consider this sequence of actions:
Mario starts transaction and inserts a new row for user 4.
Bill starts transaction and inserts a new row for user 4.
Mario's session fires a trigger to computes MAX(id)+1 for user 4. You get 3.
Bill's session fires a trigger to compute MAX(id). I get 3.
Bill's session finishes his INSERT and commits.
Mario's session tries to finish his INSERT, but the row with (userid=4, id=3) now exists, so Mario gets a primary key conflict.
In general, you can't control the order of execution of these steps without some kind of synchronization.
The solutions to this are either:
Get an exclusive table lock. Before trying an INSERT, lock the table. This is necessary to prevent concurrent INSERTs from creating a race condition like in the example above. It's necessary to lock the whole table, since you're trying to restrict INSERT there's no specific row to lock (if you were trying to govern access to a given row with UPDATE, you could lock just the specific row). But locking the table causes access to the table to become serial, which limits your throughput.
Do it outside transaction scope. Generate the id number in a way that won't be hidden from two concurrent transactions. By the way, this is what AUTO_INCREMENT does. Two concurrent sessions will each get a unique id value, regardless of their order of execution or order of commit. But tracking the last generated id per userid requires access to the database, or a duplicate data store. For example, a memcached key per userid, which can be incremented atomically.
It's relatively easy to ensure that inserts get unique values. But it's hard to ensure they will get consecutive ordinal values. Also consider:
What happens if you INSERT in a transaction but then roll back? You've allocated id value 3 in that transaction, and then I allocated value 4, so if you roll back and I commit, now there's a gap.
What happens if an INSERT fails because of other constraints on the table (e.g. another column is NOT NULL)? You could get gaps this way too.
If you ever DELETE a row, do you need to renumber all the following rows for the same userid? What does that do to your memcached entries if you use that solution?
SQL Server should allow you to do this. If you can't implement this using a computed column (probably not - there are some restrictions), surely you can implement it in a trigger.
MySQL also would allow you to implement this via triggers.
In a comment you ask the question about efficiency. Unless you are dealing with extreme volumes, storing an 8 byte DATETIME isn't much of an overhead compared to using, for example, a 4 byte INT.
It also massively simplifies your data inserts, as well as being able to cope with records being deleted without creating 'holes' in your sequence.
If you DO need this, be careful with the field names. If you have uid and id in a table, I'd expect id to be unique in that table, and uid to refer to something else. Perhaps, instead, use the field names property_id and amendment_id.
In terms of implementation, there are generally two options.
1). A trigger
Implementations vary, but the logic remains the same. As you don't specify an RDBMS (other than NOT MS/Oracle) the general logic is simple...
Start a transaction (often this is Implicitly already started inside triggers)
Find the MAX(amendment_id) for the property_id being inserted
Update the newly inserted value with MAX(amendment_id) + 1
Commit the transaction
Things to be aware of are...
- multiple records being inserted at the same time
- records being inserted with amendment_id being already populated
- updates altering existing records
2). A Stored Procedure
If you use a stored procedure to control writes to the table, you gain a lot more control.
Implicitly, you know you're only dealing with one record.
You simply don't provide a parameter for DEFAULT fields.
You know what updates / deletes can and can't happen.
You can implement all the business logic you like without hidden triggers
I personally recommend the Stored Procedure route, but triggers do work.
It is important to get your data types right.
What you are describing is a multi-part key. So use a multi-part key. Don't try to encode everything into a magic integer, you will poison the rest of your code.
If a record is identified by (entity_id,version_number) then embrace that description and use it directly instead of mangling the meaning of your keys. You will have to write queries which constrain the version number but that's OK. Databases are good at this sort of thing.
version_number could be a timestamp, as a_horse_with_no_name suggests. This is quite a good idea. There is no meaningful performance disadvantage to using timestamps instead of plain integers. What you gain is meaning, which is more important.
You could maintain a "latest version" table which contains, for each entity_id, only the record with the most-recent version_number. This will be more work for you, so only do it if you really need the performance.
i have 2 sql tables of a script that i need to be sync to another, this can be done with php cron (this was my plan) exept from one row
Table 1 Table 2
row 1 <----> Row 1
Row 2 <----> row 2
row 3 no sync row 3
both databases on same server
and the same user has full rights for both
i am looking for a php code to do this via a cpanel cron
on an after thought would it be best to merge the two so both updates with new data?
the issue is that in the example above i am needing row 3 to not change on both databases
I am very noob so please be nice lol Thx in advance
Update *
i should learn how to explain a bit better.
both the databases are control panels for sites, one of the tables rows has the system url in it, so if i share the database "site 2" links refers back to "site 1" this is a complex problem for me as i am very new to this.
what i need is to keep both databases upto date except that single row which in turn be different for both databases.
i have not tried anything just yet as i wouldn't know where to start :( lol
You dont have to use cron. MySQL in current version supports TRIGGERS and EVENTS.
You can use TRIGGER to copy data to another table. That copy (or any other operation) may be triggered by some event (like insert, update or delete in table). TRIGGER may run any query or any PL/SQL code.
Other option is an EVENT. This is something like internal task sheduler built in MySQL. It can also run queries, or any PL/SQL code, but it is triggered by system time (like Linux Cron). It has many advantages compared to cron.
PL/SQL is procedural SQL, with loops, variables and more.
If you think you are "noob" - i have cure for you. Read books about MySQL or if you are lazy - watch some tutorials ( http://thenewboston.org , http://phpacademy.org ).
Nobody here will write code for you. We can only fix a bug, give advice etc. :)
EDIT.
Example of EVENT:
-- this is comment in SQL language (line starts with --)
CREATE EVENT event_daily_copy_something
ON SCHEDULE
EVERY 1 DAY
COMMENT 'This text will appear in MySQL Workbench as description of event'
DO
BEGIN
INSERT INTO your_db_name.target_table_name(id, field)
SELECT id, something
FROM your_db_name.source_table_name
WHERE id = 3;
END
Synchronization of tables is quite complicated. I think you need few operations in event.
Check for new rows and copy
Check for deleted rows and delete them in "copy" table
Check for changed rows (here trigger on source table would be very useful, because trigger "knows" what row is edited and you can access new field values in table 1 and use them to update table 2).
One of MySQL tutorials - thenewboston#youtube.
I need to insert data into DB in two language, and I am having a bit of a dilemma (data needs to exist in both languages). Is it better to make user insert data in both language at once, or is it better for the user to first insert in one language and then to insert in the second one? And if the latter is better how is the most efficient way to do this? How can I present all articles that are not inserted in both language?
DB structure for the articles:
Common table for all article (same data):
**article -> id_article | image | date_created | category_id | subcategory_id**
Table where data is different:
article_info -> article_id | name | text | lang_id
If the data must exist in both languages - i.e., the application assumes that if an item exists in one language, than it must exist in the other - then you should design your application so that the user must add them both at once.
When you perform the database writes, you should also be using transactions. This will ensure that either all of your writes succeed, or none of them do. It prevents the database from being left in an indeterminate state with a record for one language but not the other.
Have a look at this CodeIgniter manual page on transactions to get an idea on how they work.
You can also use the insert_batch method in the database class to insert both records at once. I don't know how it works with all database drivers, but the mysqli driver will generate a single query when you use insert_batch, so the entire insert will succeed or the entire insert will fail, similar to what happens with transactions. That said, I would still wrap the call to insert_batch in a transaction block just to be a bit paranoid and future-proof.
I'm using PHP and MySQL to call a couple of commands. There is a group-employee cross-reference table, and employees may be added or removed from the group. I'm writing this feature by first removing all the employees and then adding back in each employee that is in the table if the group is modified. But I want to make sure that all the commands go through or else have the table roll back. How can I make sure the commands happen atomically?
You're looking for Transactions unless I'm mistaken.
Here's a great place to start: http://dev.mysql.com/doc/refman/5.0/en/sql-syntax-transactions.html
If you're using the MyIsam engine instead of InnoDB, then you can't take advantage of transactions. In that case, your best bet is to perform all your INSERTS firsts (your cross-ref table should have a two-field unique constraint so dups will silently fail). Then perform a single DELETE:
DELETE FROM employees_groups
WHERE group_id = <groupid>
AND employee_id NOT IN (list of employee ids you just inserted)
Assuming your target tables support transactions, then you want the mysqli transaction family of methods.
See:
mysqli->commit()
mysqli->autocommit()
mysqli->rollback()
If your tables do not support transactions, you could likely change your table types to 'InnoDB` without a problem, and thus support transactions. Check with your DB admin before doing so first.
I have four DB tables in an Oracle database that need to be rewritten/refreshed every week or every month. I am writing this script in PHP using the standard OCI functions, that will read new data in from XML and refresh these four tables. The four tables have the following properties
TABLE A - up to 2mil rows, one primary key (One row might take max 2K data)
TABLE B - up to 10mil rows, one foreign key pointing to TABLE A (One row might take max 1100 bytes of data)
TABLE C - up to 10mil rows, one foreign key pointing to TABLE A (One row might take max 1100 bytes of data)
TABLE D - up to 10mil rows, one foreign key pointing to TABLE A (One row might take max 120 bytes of data)
So I need to repopulate these tables without damaging the user experience. I obviously can't delete the tables and just repopulate them as it is a somewhat lengthy process.
I've considered just a big transaction where I DELETE FROM all of the tables and just regenerate them. I get a little concerned about the length of the transaction (don't know yet but it could take an hour or so).
I wanted to create temp table replicas of all of the tables and populate those instead. Then I could DROP the main tables and rename the temp tables. However you can't do the DROP and ALTER table statements within a transaction as they always do an auto commit. This should be able to be done quickly (four DROP and and four ALTER TABLE statements), but it can't guarantee that a user won't get an error within that short period of time.
Now, a combination of the two ideas, I'm considering doing the temp tables, then doing a DELETE FROM on all four original tables and then and INSERT INTO from the temp tables to repopulate the main tables. Since there are no DDL statements here, this would all work within a transaction. Then, however, I wondering if the memory it takes to process some 60 million records within a transaction is going to get me in trouble (this would be a concern for the first idea as well).
I would think this would be a common scenario. Is there a standard or recommended way of doing this? Any tips would be appreciated. Thanks.
You could have a synonym for each of your big tables. Create new incarnations of your tables, populate them, drop and recreate the synonyms, and finally drop your old tables. This has the advantage of (1) only one actual set of DML (the inserts) avoiding redo generation for your deletes and (2) the synonym drop/recreate is very fast, minimizing the potential for a "bad user experience".
Reminds me of a minor peeve of mine about Oracle's synonyms: why isn't there an ALTER SYNONYM command?
I'm assuming your users don't actually modify the data in these tables since it is deleted from another source every week, so it doesn't really matter if you lock the tables for a full hour. The users can still query the data, you just have to size you rollback segment appropriately. A simple DELETE+INSERT therefore should work fine.
Now if you want to speed things up AND if the new data has little difference with the previous data you could load the new data into temporary tables and updating the tables with the delta with a combination of MERGE+DELETE like this:
Setup:
CREATE TABLE a (ID NUMBER PRIMARY KEY, a_data CHAR(200));
CREATE GLOBAL TEMPORARY TABLE temp_a (
ID NUMBER PRIMARY KEY, a_data CHAR(200)
) ON COMMIT PRESERVE ROWS;
-- Load A
INSERT INTO a
(SELECT ROWNUM, to_char(ROWNUM) FROM dual CONNECT BY LEVEL <= 10000);
-- Load TEMP_A with extra rows
INSERT INTO temp_a
(SELECT ROWNUM + 100, to_char(ROWNUM + 100)
FROM dual
CONNECT BY LEVEL <= 10000);
UPDATE temp_a SET a_data = 'x' WHERE mod(ID, 1000) = 0;
This MERGE statement will insert the new rows and update the old rows only if they are different:
SQL> MERGE INTO a
2 USING (SELECT temp_a.id, temp_a.a_data
3 FROM temp_a
4 LEFT JOIN a ON (temp_a.id = a.id)
5 WHERE decode(a.a_data, temp_a.a_data, 1) IS NULL) temp_a
6 ON (a.id = temp_a.id)
7 WHEN MATCHED THEN
8 UPDATE SET a.a_data = temp_a.a_data
9 WHEN NOT MATCHED THEN
10 INSERT (id, a_data) VALUES (temp_a.id, temp_a.a_data);
Done
You will then need to delete the rows that aren't in the new set of data:
SQL> DELETE FROM a WHERE a.id NOT IN (SELECT temp_a.id FROM temp_a);
100 rows deleted
You would insert into A then into the child tables and deleting in reverse order.
Am I the only one (except Vincent) who would first test the simplest possible solution, i.e. DELETE/INSERT, before trying to build something more advanced?
Then, however, I wondering if the memory it takes to process some 60 million records within a transaction is going to get me in trouble (this would be a concern for the first idea as well).
Oracle manages memory quite well, it hasn't been written by a bunch of Java novices (oops it just came out of my mouth!). So the real question is, do you have to worry about the performance penalties of thrashing REDO and UNDO log files... In other words, build a performance test case and run it on your server and see how long it takes. During the DELETE / INSERT the system will be not as responsive as usual but other sessions can still perform SELECTs without any fears of deadlocks, memory leaks or system crashes. Hint: DB servers are usually disk-bound, so getting a proper RAID array is usually a very good investment.
On the other hand, if the performance is critical, you can select one of the alternative approaches described in this thread:
partitioning if you have the license
table renaming if you don't, but be mindful that DDLs on the fly can cause some side effects such as object invalidation, ORA-06508...
In Oracle your can partition your tables and indexes based on a Date or time column that way to remove a lot of data you can simply drop the partition instead of performing a delete command.
We used to use this to manage monthly archives of 100 Million+ records and not have downtime.
http://www.oracle.com/technology/oramag/oracle/06-sep/o56partition.html is a super handy page for learning about partitioning.
I assume that this refreshing activity is the only way of data changing in these tables, so that you don't need to worry about inconsistencies due to other writing processes during the load.
All that deleting and inserting will be costly in terms of undo usage; you also would exclude the option of using faster data loading techniques. For example, your inserts will go much, much faster if you insert into the tables with no indexes, then apply the indexes after the load is done. There are other strategies as well, but both of them preclude the "do it all in one transaction" technique.
Your second choice would be my choice - build the new tables, then rename the old ones to a dummy name, rename the temps to the new name, then drop the old tables. Since the renames are fast, you'd have a less than one second window when the tables were unavailable, and you'd then be free to drop the old tables at your leisure.
If that one second window is unacceptable, one method I've used in situations like this is to use an additional locking object - specifically, a table with a single row that users would be required to select from before they access the real tables, and that your load process could lock in exclusive mode before it it does the rename operation.
Your PHP script would use two connections to the db - one where you do the lock, the other where you do the loading, renaming and dropping. This way the implicit commits in the work connection won't terminate the lock in the other table.
So, in the script, you'd do something like:
Connection 1:
Create temp tables, load them, create new indexes
Connection 2:
LOCK TABLE Load_Locker IN SHARE ROW EXCLUSIVE MODE;
Connection 1:
Perform renaming swap of old & new tables
Connection 2:
Rollback;
Connection 1:
Drop old tables.
Meanwhile, your clients would issue the following command immediately after starting a transaction (or a series of selects):
LOCK TABLE Load_Locker IN SHARE MODE;
You can have as many clients locking the table this way - your process above will block behind them until they have all released the lock, at which point subsequent clients will block until you perform your operations. Since the only thing you're doing inside the context of the SHARE ROW EXCLUSIVE lock is renaming tables, your clients would only ever block for an instant. Additionally, putting this level of granularity allows you to control how long the clients would have a read consistent view of the old table; without it, if you had a client that did a series of reads that took some time, you might end up changing the tables mid-stream and wind up with weird results if the early queries pulled old data & the later queries pulled new data. Using SET TRANSACTION SET ISOLATION LEVEL READ ONLY would be another way of addressing this issue if you weren't using my approach.
The only real downside to this approach is that if your client read transactions take some time, you run the risk of other clients being blocked for longer than an instant, since any locks in SHARE MODE that occur after your load process issues its SHARE ROW EXCLUSIVE lock will block until the load process finishes its task. For example:
10:00 user 1 issues SHARE lock
10:01 user 2 issues SHARE lock
10:03 load process issues SHARE ROW EXCLUSIVE lock (and is blocked)
10:04 user 3 issues SHARE lock (and is blocked by load's lock)
10:10 user 1 releases SHARE
10:11 user 2 releases SHARE (and unblocks loader)
10:11 loader renames tables & releases SHARE ROW EXCLUSIVE (and releases user 3)
10:11 user 3 commences queries, after being blocked for 7 minutes
However, this is really pretty kludgy. Kinlan's solution of partitioning is most likely the way to go. Add an extra column to your source tables that contains a version number, partition your data based on that version, then create views that look like your current tables that only show data that shows the current version (determined by the value of a row in a "CurrentVersion" table). Then just do your load into the table, update your CurrentVersion table, and drop the partition for the old data.
Why not add a version column? That way you can add the new rows with a different version number. Create a view against the table that specifies the current version. After the new rows are added recompile the view with the new version number. When that's done, go back and delete the old rows.
What we do in some cases is have two versions of the tables, say SalesTargets1 and SalesTargets2 (an active and inactive one.) Truncate the records from the inactive one and populate it. Since no one but you uses the inactive one, there should be no locking issues or impact on the users while it is populating. Then have view that selcts all the information from the active table (it should be named what the current table is now, say SalesTargets in my example). Then to switch to the refreshed data, all you have to do is run an alter view statement.
Have you evaluated the size of the delta (of changes).
If the number of rows that get updated (as opposed to inserted) every time you put up a new rowset it not too high, then I think you should consider importing the new set of data into a set of staging tables and do an update-where-exists and insert-where-not-exists (UPSERT) solution and just refresh your indexes (ok ok indices).
Treat it like ETL.
I'm going with an upsert method here.
I added an additional "delete" column to each of the tables.
When I begin processing the feed, I set the delete field for every record to '1'.
Then I go through a serious of updates if the record exists, or inserts if it does not. For each of those inserts/updates, the delete field is then set to zero.
At the end of the process I delete all records that still have a delete value of '1'.
Thanks everybody for your answers. I found it very interesting/educational.