Quick Tip: How to Take Snapshots of Data at Specified Intervals
A few weeks ago I was debugging something and needed to take snapshots of a table every five seconds, as a quick solution a colleague started running the following SQL and I thought I’d share it for anyone else stuck in a pinch…
WHILE (1=1) BEGIN INSERT INTO tmp_snapshot SELECT field1,field2,field3 FROM table1 WHERE field4 = 'value' WAITFOR DELAY '000:00:05' END
Just change the time delay and the query and you’re off, of course you could be a lot cleverer with your WHILE conditions and rework this principle for loads of things. I thought it worth posting as many database folk forget the power of the humble while loop!
Categories: DBA, Microsoft SQL Server Tags: interval, loop, Microsoft SQL Server, snapshot, sql, SQL Server, T-SQL, TSQL, while
Update a Table Using a Join in MySQL
It’s one of those occasional requirements that come up in a DBA or Data Warehouse architects’s life, to update one table with values from another. It’s a fairly straight-forward syntax in MySQL but one that differs from my ‘platform of choice’ Microsoft SQL Server, the syntax goes…
UPDATE child c INNER JOIN parent p ON p.id_parent = c.id_parent SET c.parent_name = p.name WHERE c.parent_name IS NULL AND p.legal_guardian = 1;
Now my example is fairly contrived but it does show not only updating the table over the join but also using criteria in the WHERE clause, there’s also an advantage over T-SQL in that you can use the table alias in all cases.
Change a Column’s Data Type in MySQL (equivalent of ALTER COLUMN)
I just thought I’d post this little tip as the syntax difference between MySQL and Microsoft SQL Server
just foxed me once again…
ALTER TABLE my_table CHANGE my_column my_column SMALLINT NOT NULL DEFAULT 0;
You may wonder why the column name is repeated twice, that’s because the same syntax also
allows you to rename the column at the same time. The syntax might be a little odd but it does makes sense if you think about it.
The Database Countryside Code: Best Practices for BI & SQL Users
Those ‘City Folk’ among you may not be aware but in Rural England we have what is called The Countryside Code, it’s a set of guidelines that everyone should follow in order to keep the countryside clean, tidy and a nice place to visit. You may be asking – what does this have to do with Business Intelligence and Database Administration? Well, I think it’s vital – if we all follow a fairly simple but broad set of guidelines then all classes of database user will have a better experience from Developers to DBAs and Analysts to CIOs. This isn’t really about making your databases perform better, it’s about working better with each-other and taking other people’s perspectives on board. Having been in most of the related roles over the years this is what I’d put into The Database Countryside Code…
1. Enjoy the countryside and respect its life and work
Whether your application is an ‘out of the box’ software suite, a Business Intelligence package that can be tweaked on implementation or a hand-crafted bespoke solution if you’re running against a database maintained by someone else or shared with other applications you need to take heed of this point. Remember that cooperation is key and if you build a good relationship with the DBA and the other key users of the database you’ll have a much better time of things and if there are any critical issues you’ll be included in the remediation process and may even be able to help your own users get back online faster. It’s easy to see DBAs as grouchy, narrowly focused sorts who tend to view all user activity as bothersome (I can say that as I’ve been one myself) but generally speaking if the DBA is aware of user activity at all the chances are that there’s already a problem as it’s the long running, resource intensive activity that will stand out in alerts and performance reports. Before your application goes live you should do some testing, run your designs and SQL statements / stored procedures past the DBA for some advice (but remember, you don’t have to take it) and establish some sort of procedure for reporting issues, and remember that an SLA can work both ways as you may need the DBA’s help as much as they might need yours.
2. Guard against all risk of fire
Security is a huge issue and as exploit frameworks and toolkits become more and more prevalent and feature-rich the likelihood of vulnerabilities being discovered in our applications should be treated more like a certainty. If you’re developing bespoke applications and especially web apps you’ll need to pay close attention to the OWASP Top 10 application security risks but from a database perspective the most notable threat is SQL Injection - the art of passing SQL into an application so that it might be executed by the database (as a good starting point check out OWASP’s SQL Injection Prevention Cheat Sheet). If you’re deploying packaged apps or BI tools don’t think that you’ve gotten away with it, the primary responsibility may be on software developers to avoid exploits but if they’re baked into an application you’re implementing it will affect your users and your business, so…
3. Protect wildlife, plants and trees
The most important security contribution we as implementers can bring to the table is to review and limit the privileges required by our applications. Many install guides and expensive external consultants ask for a ‘dbo’ (database owner) level user and some even ask for ‘sa’ (system administrator) or ‘root’ level privileges but don’t hand these out like candy on halloween. In most cases these high-level privileges are only required during setup and install and can be removed afterwards but often basic read/write access is all that is required (and for BI tools often read-only), it may only be achievable through a few frustrating rounds of trial and error but if you assign your applications the lowest possible permissions you will significantly reduce the risk of compromise in the future. Another important step during implementation is to make sure that your permissions are segregated, where possible have a separate user for each service and an entirely separate user for accessing each database not shared by any other application. Whilst it may seem excessive this setup will allow you to audit any security issues and identify which user was compromised and exactly what they had access to.
4. Fasten all gates
Many Business Intelligence tools include some degree of control over connection management and if you’re developing your own application you’ll have complete control over all database connections, the decision to be made is whether connections are ‘pinned’ open, closed after x minutes or closed at the end of each transaction. The preference will vary depending on the load and the usage, in most Business Intelligence use cases there tend to be a large number of users, not always connecting concurrently and issuing fairly large queries against the database followed by periods of quiet whilst a report is read – in this case there is usually no need to keep the connection open for long. On the other hand if you have users issuing a constant stream of small transactions (e.g. a Point of Sale system) the overhead of creating and dropping connections might actually add load to the database so it would be more effective in this scenario to maintain the connection.
5. Keep your dogs under close control
This applies more to developers and BI architects where your dogs are your users, if you are deploying an application that creates load on somebody else’s database you should do whatever you can to limit each user’s ability to cause long running queries – in some BI tools you are handed an option to let a query time out after x minutes and perhaps limiting the number of rows returned. If you are developing your own application you should include both of these options but make sure that you kill the query at the database level rather than just killing the thread in your application that made the request otherwise it’s equally bad if not worse since the user may simply re-issue the offending query. The actual limits are bound to vary from database to database but that’s where the first point comes in, discuss this with both your users and the DBA.
6. Keep to public paths across farmland / Use gates and stiles to cross fences, hedges and walls
When it comes to solving problems try to stick within the basic and simple boundaries of an ordinary user, avoid using undocumented stored procedures, excessive use of user defined functions, custom data types, plugins and extended stored procedures or anything else that strays too far from a standard install of the database platform. Obviously you’ve got an app to deploy and you want to solve your problems in whatever way is best for your users but the further you are from a standard deployment the more issues you’re likely to encounter, both you and the DBA might be fully aware of this amazing new setting you tweaked to make things run better but a couple of years down the line during a disaster recovery will it all come flooding back quite as easily? What if one or both of you that setup the application have moved on to other roles? Thinking outside the box is great but be conscious of introducing risk and if you do feel that it is necessary then make sure that it’s well documented in the Run Book or the corporate wiki.
7. Leave livestock, crops and machinery alone
Since you may already have elevated privileges on your own database, a shared database or even the server you may be tempted from time to time to perform maintenance tasks or make minor ‘improvements’ to indexes or configuration settings – do not do so without the DBA’s blessing. If you’re following the rules above you’ll probably have a fairly good rapport with the DBA already so it’s likely that you’ll be granted some level of trust not to mess things up but be careful not to overreach, the DBA will be ‘in the loop’ of many changes and other requirements (e.g. critical deadlines, disaster recovery tests, unplanned maintenance) whereas you may not be aware of them so before you make any changes run them past the DBA – just in case.
8. Take your litter home / Help to keep all water clean
If you’ve ever been a DBA you’ll have seen, on more than one occasion, tables popping up called tmpSomethingorOther, tblToBeDeleted or TableName_bak but when it comes to the key questions (How long have these been around? Are they still required?) nobody seems to have a straight answer. I know myself that whilst I’ve been developing data warehouses I’ve created these sorts of tables and subsequently forgotten what they were used for, not too much of a problem if you’re ‘the guy’ but in a large team or with personnel changes over time it can be hard to know what is required and what isn’t – I came to a database once with temporary tables over five years old which had not been deleted out of fear that they were important. The moral here is an obvious one, clean up after yourself or if the table must exist for some short period of time put a note in your diary to come back and cull it.
9. Make no unnecessary noise
Be mindful of what errors you raise and what you write to public logs, if your application causes a large amount of data to be written to database or other centrally collated logs you may inadvertently make it harder to detect genuine issues which will hurt both you and and other users of the database. If you do occasionally need exhaustive logs consider adding a ‘debug mode’ into your application which can be turned on or off via a configuration setting, that way you can turn it on whilst you’re tracing a fault and need more verbose logging then turn it off when you’re done.
10. Take special care on country roads
There can be plenty of unexpected hazards on country roads so don’t always rush around everywhere at 60mph, acknowledge that whist you might want everything to go as fast as possible you could be causing some other critical process to slow or stop. Driving at night can be treacherous too as you might come across an unexpected backup window or import/export process, talk to your DBA and coordinate the major tasks. If it’s a shared server make sure you have access to the task list so that you know where to slot in your jobs and that those jobs get put back into the master list.
Really it comes down to one thing, as the great and wise Jerry Springer oft said, “take care of yourselves, and each other”.
Categories: Business Intelligence, DBA Tags: BI, DBA, logging, performance, Security, sql
Quick Tip – PostgreSQL Equivalent of ISNUMERIC()
Very much like my previous MySQL ISNUMERIC() post I have recently been setting up a data source to collect records with telephone numbers from a Postgres database and one of the essential validation tests is to make sure that the field really does contain a number.
Despite the fact that many regard Postgres as the best open source database platform I find myself frustrated by it’s lack of standard functions. I understand that Postgres is designed to be extensible and that user defined functions can be built but I need my code to be both portable and read-only so I have to work with what I’m given. Ideally what I’d be looking for is an equivalent of Microsoft SQL Server’s ISNUMERIC() or Excel’s ISNUMBER() functions but very much like MySQL I had to turn to regular expressions although as you’ll see, Postgres does not have a clean and clear REGEXP() function…
SELECT DISTINCT contact_number
FROM customers
WHERE (contact_number ~ ‘^[0-9]+$’)
I hope that helps any of you out there that encounter the same problem, thanks to the poster here for my original answer.
Categories: DBA, PostgreSQL Tags: ISNUMERIC, MySQL, Postgres, PostgreSQL, RegExp, Regular Expressions, sql
Using MySQL BLOB Data via ODBC in SSIS, SQL Server & Business Objects
Whilst trying to build a centralised cross-platform alerting system I spotted a peculiar issue when trying to move the output of a SHOW FULL PROCESSLIST command on MySQL via ODBC. It seems that the output of the SHOW FULL PROCESSLIST command returns both integers and binary (BLOB) data types even though to they eye (that is, in the MySQL Query Browser) most of the columns appear to be short text fields.
Despite the fact that the data looks like text whenever I tried to return the data into an application, I tried SSIS, SQL Server Linked Servers and Business Objects, each time the data would come back unusable or an error would be returned. Business Objects gave me the key by declaring “This is a BLOB.” as you can see in the following screenshots…
SQL Server Integration Services
SQL Server Linked Server
Business Objects Desktop Intelligence
As with my recent post about loading data into MySQL with SSIS the saviour turns out to be an ODBC configuration setting, this time in the Metadata tab of the MySQL ODBC driver. All you have to do is check the “Always handle binary function results as character data” and instantly your problems will be solved…
Out of a crazy fit of completeness I also took screenshots of the final results and it’d be a shame to waste them so here they are…
SQL Server Integration Services
SQL Server Linked Server
Business Objects Desktop Intelligence
Categories: Business Objects, DBA, Microsoft SQL Server, MySQL, Open Source, SSIS Tags: binary, blob, business objects, DBA, ETL, Integration Services, mys, MySQL, ODBC, SQL Server, SSIS