In MySQL, which is a better practice? Always use "CREATE TABLE IF NOT EXISTS", or check first the existence of the table using "SHOW TABLES LIKE" before making the table?
I have to regularly save a page where the table for it may or may not be there (sometimes, it is deliberately deleted when not in use). Previously, I used to do "SHOW TABLES LIKE" to check if that table exists before I insert new entries. But I changed it to "CREATE TABLE IF NOT EXISTS". Either way, I just do a "INSERT .. ON DUPLICATE UPDATE" to add new or update existing data.
I don't know how to benchmark this, which is why I am asking.
Performance isn't critical with these operations.
The key aspect is race conditions. If you use CREATE TABLE IF NOT EXISTS you know it will happen or it won't. If two threads happen to be doing this statement one will succeed and the other won't be negatively affected.
If a SHOW TABLE LIKE was used in both threads, both could detect the table didn't exist, and upon trying to create the table, one would fail.
So use CREATE TABLE IF NOT EXISTS to mitigate race conditions. Also in general is better to use a database provided feature than roll your own.
CREATE TABLE IF NOT EXISTS is option provided by MySQL and good to use. If table will not exist this statement will create else will skip.
other end, if you first check table existence by show table like then either skip or create as per result of condition/check. Ultimately you are increasing one step or runtime of you script or program for same functionality which you can achieve in single step.
Related
Is there any way to detect when an ALTER TABLE statement is executed in MySQL? For example, if the following statement were executed on some_table, is there any way to detect that the column name changed from column_name_a to column_name_b and log it in another table in the DB?
ALTER TABLE `some_table`
CHANGE COLUMN `column_name_a` `column_name_b` VARCHAR(255) NULL DEFAULT NULL;
Thanks.
To my knowledge it is unfortunately not possible to put triggers on the INFORMATION_SCHEMA tables, since they are strictly spoken views and triggers can't be made to work on views. If triggers would be possible on the INFORMATION SCHEMA, then you could have a trigger on updates of the INFORMATION_SCHEMA.COLUMNS table to identify name changes.
However, what you can do is one of the following things:
option 1) Maintain a real table with all column names. Then create a function that checks for a discrepancy between the INFORMATION_SCHEMA.COLUMNS table abd your table. If there is one, you know the name has changed. You need to copy over the new name to your column name table and do whatever else you wanted to do upon name change.
The function to check for discrepancies then must be run periodically via the mysql scheduler in order to detect name changes as quickly as possible. Note that this is not a real time solution. There will be a lag between the ÀLTER TABLE command and its detection. If this is unacceptable in your scenario you need to go with
option 2) Do not call ÀLTER TABLE directly, but wrap it in a function. Within this function you can also call other functions to achieve what you need to achieve. If may be worth while to formulate the needed steps in a higher programming language that you use to drive your application. If this is not possible, you will be limited to the possibilities that are offered in functions/procedures in the mysql environment.
Sorry to not have a simpler way of doing this for you.
I have recently installed a new computer with Percona Server 5.6 instead of MySQL 5.6, and using InnoDB/XtraDB mostly, FWIW. The database I'm working on is merely a testing ground, but I have 1 issue: after I add a column to a table (or even remove one), I usually forget to INSERT or otherwise change another table's data, which keeps track of what column names are in which table; each table has ASCII name along with a number, and this number is the only difference between table names for simplicity. So, is there a way to auto-update the "relation" table so that the column name and table's number are added or changed, instead of using a cronjob ?
Now that I think, I could DROP that table and use information_schema instead ...
EDIT 0: Don't let the above realization stop you; it's just good to know if this is possible before going for a possible other way.
Yes, relying on the 'INFORMATION_SCHEMA.COLUMNS' may be best.
Unfortunately mysql does not support DDL TRIGGER events, as this would be what you are looking for.
triggers allow you to perform many SQL and procedural operations before insertion, update or deletion of rows in a specific table. However to the best of my knowledge - and I would be stoked if I were wrong - you cant set TRIGGER events on DDL statements like ALTER and DROP TABLE...
However still take the time to learn about triggers - they save a lot of time by eliminating the need for cronjobs and external updates for things like aggregate values.
https://dev.mysql.com/doc/refman/5.6/en/trigger-syntax.html
Is there a way to do this?
In case the DBMS command history got cleaned or, in my case, when many ALTER TABLE were used in the course of time.
I'm using MySQL.
Yes, it is as simple as
SHOW CREATE TABLE yourtable;
This will include all the subsequent ALTER TABLE statements. You cannot retrieve the table's original state.
Here is the relevant documentation
Once a record is entered in a particular table, I think I don't ever want it to be deleted. The records are there to keep track of things for historical purposes. I'm using MySQL, InnoDB. Is there any way I can protect that table from accidental deletes by the application layer? If so, is this a good approach?
If you can set the permission for your user(s), don't give the delete permission. There're situations where this practice is a "must be" like the tables for statistics purpose. Thus, if your table is used to achieve one of this goal, it's a good approach.
I use a trigger that detects the deletion and does some illegal stuff so the whole operation fails. For example like this:
CREATE TRIGGER protect_delete before delete ON protected_table
FOR EACH ROW UPDATE non_existing_table SET non_existing_column=1;
So when someone will attempt a delete operation - the whole statement will fail. You also might use better names for non_existing_column and non_existing_table.
E.g it is possible to obtain an error message like this:
ERROR 1146 (42S02): Table 'database.delete_restricted_on_tableX'
doesn't exist
EDIT: also it is possible to create even better fail messages, please check here http://www.brokenbuild.com/blog/2006/08/15/mysql-triggers-how-do-you-abort-an-insert-update-or-delete-with-a-trigger/
One other option is switch to the ARCHIVE engine for historical tables.
This will prevent any DELETE or UPDATE actions on the table, and compress the data. One (major) downside to this approach is that you cannot index the columns on the table.
I think you should implement this logic in your application layer and insert a column where you put a no-delete flag.
Another idea would be to exclude the delete access for the db user
You may want to write a trigger that detects the deletion, then reinserts the record, but there may be some issues with that, so you can also just add a foreign key constraint that will prevent the deletion.
For some discussions on this you can look at: http://rpbouman.blogspot.com/2011/10/mysql-hacks-preventing-deletion-of.html.
Here is a chunk of the SQL I'm using for a Perl-based web application. I have a number of requests and each has a number of accessions, and each has a status. This chunk of code is there to update the table for every accession_analysis that shares all these fields for each accession in a request.
UPDATE accession_analysis
SET analysis_id = ? ,
reference_id = ? ,
status = ? ,
extra_parameters = ?
WHERE analysis_id = ?
AND reference_id = ?
AND status = ?
AND extra_parameters = ?
and accession_id is (
SELECT accesion_id
FROM accessions
where request_id = ?
)
I have changed the tables so that there's a status table for accession_analysis, so when I update, I update both accession_analysis and accession_analysis_status, which has status, status_text and the id of the accession_analysis, which is a not null auto_increment variable.
I have no strong idea about how to modify this code to allow this. My first pass grabbed all the accessions and looped through them, then filtered for all the fields, then updated. I didn't like that because I had many connections with short SQL commands, which I understood to be bad, but I can't help but think the only way to really do this is to go back to the loop in Perl holding two simpler SQL statements.
Is there a way to do this in SQL that, with my relative SQL inexperience, I'm just not seeing?
The answer depends on which DBMS you're using. The easiest way is to create a trigger on one table that provides the logic of updating the other table. (For any DB newbies -- a trigger is procedural code attached to a table at the DBMS (not application) layer that runs in response to an insert, update or delete on the table.). A similar, slightly less desirable method is to put the logic in a stored procedure and execute that instead of the update statement you're now using.
If the DBMS you're using doesn't support either of these mechanisms, then there isn't a good way to do what you're after while guaranteeing transactional integrity. However if the problem you're solving can tolerate a timing difference in the two tables' updates (i.e. The data in one of the tables is only used at predetermined times, like reporting or some type of batched operation) you could write to one table (live) and create a separate process that runs when needed (later) to update the second table using data from the first table. The correctness of allowing data to be updated at different times becomes a large and immovable design assumption, however.
If this is mostly about connection speed, then one option you have is to write a stored procedure that handles the "double update or insert" transparently. See the manual for stored procedures:
http://dev.mysql.com/doc/refman/5.5/en/create-procedure.html
Otherwise, You probably cannot do it in one statement, see the MySQL INSERT syntax:
http://dev.mysql.com/doc/refman/5.5/en/insert.html
The UPDATE syntax allows for multi-table updates (not in combination with INSERT, though):
http://dev.mysql.com/doc/refman/5.5/en/update.html
Each table needs its own INSERT / UPDATE in the query.
In fact, even if you create a view by JOINing multiple tables, when you INSERT into the view, you can only INSERT with fields belonging to one of the tables at a time.
The modifications made by the INSERT statement cannot affect more than one of the base tables referenced in the FROM clause of the view. For example, an INSERT into a multitable view must use a column_list that references only columns from one base table. For more information about updatable views, see CREATE VIEW.
Inserting data into multiple tables through an sql view (MySQL)
INSERT (SQL Server)
Same is true of UPDATE
The modifications made by the UPDATE statement cannot affect more than one of the base tables referenced in the FROM clause of the view. For more information on updatable views, see CREATE VIEW.
However, you can have multiple INSERTs or UPDATEs per query or stored procedure.