Databases matter

Database

Databases are at the core of virtually all modern information technology systems. Sometimes these databases are exposed, for example, as in data warehouse systems. The centrepiece of a system like that is a powerful industrial database such as Oracle, or Sybase, or Microsoft SQL Server, or something similar. The presence of a database is pretty obvious in that case – only these powerful databases can crunch the immense amounts of data processed by data warehouses. In other cases databases are hidden. For example, Apple iTunes uses a SQLite database to store the details of music tracks on your computer. That database is not obvious; it does not advertise itself, but it’s there. It makes sure that the ratings you assigned to songs are saved and can be synchronised between all your iPhones and iPads. It counts the number of times every song was played so that you don’t listen to the same song twice when you put your iPhone on shuffle. Databases are everywhere. They all serve the same purpose – to store data and make its retrieval as easy and fast as possible – but they are also vastly different from each other.

Let’s take another example. When you log into your Google account and bring up your Gmail inbox, all the emails you see are actually stored in the remote database. That database is called BigTable, and it contains not only your emails, but all the emails of all Gmail users in the world, and also virtually all of Google’s data. While your iTunes SQLite database may be about 50 megabytes in size (and that’s assuming you have A LOT of songs), Google’s BigTable contains petabytes of data. That’s your iTunes database times one billion.

If you think about it, it becomes obvious that these databases require vastly different approaches to the way the data are stored and retrieved: You can fit an iTunes database into memory and query it whichever way you like without a performance penalty. At the same time, no machine has been built yet that could apply the same approach to Google’s BigTable.

Unfortunately, not all software developers understand that. Databases once were an inspiring topic but in recent years they went out of fashion. Software developers are geeks; they like new toys; they all want to work on something latest and greatest and cutting-edge. So many new exciting things are happening in the area of Information Technology – Web 2.0, HTML5 and Apple iOS to name just a few – that databases just fade in comparison, despite the fact that they make all these new shiny things tick. Most of the developers these days take a database just as generic data storage: “We’ll just stuff the data in and we don’t care what’s inside.” 10 years ago SQL language was a necessary skill for database application developers. Nowadays the majority of programmers don’t know SQL. They rely on frameworks such as Hibernate to produce SQL statements for them. They think that all databases are the same and therefore, if necessary, they can take one system that uses MS SQL Server as a backend and put it onto Oracle and it will work just fine.

Well, that may be true for very simple applications. The myth that all databases are the same is flawed, especially when it comes down to performance. Today cloud computing is a buzzword, and Google is the patriarch of the cloud. Google’s servers process billions of requests every day, crunching petabytes of data. Yet, every request made to a Google search engine is served within seconds. This places such a high demand on the Google’s database layer, that Google’s engineers couldn’t afford using even the most powerful of industrial databases, and they had to develop their own – the aforementioned BigTable. If you told these guys that “all databases are the same”, they would laugh into your face, and rightfully so, because Google knows that performance matters and they try to squeeze every bit of performance out of their systems.

Databases matter, and if you consider yourself a decent software developer, you need to learn how to tame them. Learn the differences between them. Learn what they are, what makes them tick, and the most importantly, how to make them tick faster, because writing applications that are slow is just bad taste.

Don’t mess with LIKE

As I have already shown you, implicit type conversion is one of the most dangerous features of Oracle SQL and PL/SQL. It is dangerous because it happens automatically without your knowledge and may lead to unpredictable results. These problems are most common when dealing with DATE conversions, but not limited to them.

For example, let’s have a look at this Stackoverflow.com question by James Collins. James had a problem, the following query was slow:

SELECT a1.*
FROM   people a1
WHERE  a1.ID LIKE '119%'
AND ROWNUM < 5

Despite column A1.ID was indexed, the index wasn’t used and the explain plan looked like this:

SELECT STATEMENT ALL_ROWS
Cost: 67 Bytes: 2,592 Cardinality: 4 2 COUNT STOPKEY 1 TABLE ACCESS FULL TABLE people
Cost: 67 Bytes: 3,240 Cardinality: 5

James was wondering why.

Well, the key to the issue lies, as it often happens with Oracle, in an implicit data type conversion. Because Oracle is capable to perform automatic data conversions in certain cases, it sometimes does that without you knowing. And as a result, performance may suffer or code may behave not exactly like you expect.

In our case that happened because ID column was NUMBER. You see, LIKE pattern-matching condition expects to see character types as both left-hand and right-hand operands. When it encounters a NUMBER, it implicitly converts it to VARCHAR2. Hence, that query was basically silently rewritten to this:

SELECT a1.*
FROM   people a1
WHERE  To_Char(a1.ID) LIKE '119%'
AND ROWNUM < 5

That was bad for 2 reasons:

  1. The conversion was executed for every row, which was slow;
  2. Because of a function (though implicit) in a WHERE predicate, Oracle was unable to use the index on A1.ID column.

If you came across a problem like that, there is a number of ways to resolve it. Some of the possible options are:

  1. Create a function-based index on A1.ID column:
CREATE INDEX people_idx5 ON people (To_char(ID));
  1. If you need to match records on first 3 characters of ID column, create another column of type NUMBER containing just these 3 characters and use a plain = operator on it.
  2. Create a separate column ID_CHAR of type VARCHAR2 and fill it with TO_CHAR(id). Then index it and use instead of ID in your WHERE condition.
  3. Or, as David Aldridge pointed out: “It might also be possible to rewrite the predicate as ID BETWEEN 1190000 and 1199999, if the values are all of the same order of magnitude. Or if they’re not then ID = 119 OR ID BETWEEN 1190 AND 1199, etc.”

Of course if you choose to create an additional column based on existing ID column, you need to keep those 2 synchronized. You can do that in batch as a single UPDATE, or in an ON-UPDATE trigger, or add that column to the appropriate INSERT and UPDATE statements in your code.

James choose to create a function-based index and it worked like a charm.

SYSDATE confusions

SYSDATE is one of the most commonly used Oracle functions. Indeed, whenever you need the current date or time, you just type SYSDATE and you’re done. However, sometimes it’s not all that simple. There are a few confusions associated with SYSDATE that are pretty common and, if not understood, can cause a lot of damage.

First of all, SYSDATE returns not just current date, but date and time combined. More precisely, the current date and time down to a second. If just a date is needed, TRUNC function has to be applied, that is, TRUNC(SYSDATE). For a sake of a good database design, date should not be confused with date/time. For example, if a column in a table is called transaction_date, it would be natural for it to contain a date, but not date/time. That may lead to a major confusion. Let’s imagine there is a table BANK_TRANSACTIONS containing the following fields:

txn_no     INTEGER,
txn_amount NUMBER(14,2),
txn_date   DATE

The last field is of the most interest to us. Apparently its data type is DATE, but is it a date or date/time? We can’t tell by just looking at the table definition. Nonetheless, it is a very important thing to know. A common case for using DATE columns is including them in date range queries. Forexample, if we wanted to get all the bank transactions from 1 January 2009 to 31 July 2009 we could write this:

SELECT txn_no,
       txn_amount
FROM   bank_transactions
WHERE  txn_date BETWEEN To_date('01-JAN-2009','DD-MON-YYYY')
AND    To_date('31-JUL-2009','DD-MON-YYYY')

And that would be fine if TXN_DATE were a date column. But if it is a date/time, we would just have missed a whole day worth of data. And it is because, as I said, DATE data type can hold date/time down to a second. That means that for 31 July 2009 it could hold values ranging from 0:00am to 11:59pm. But because TO_DATE('31-JUL-2009', 'DD-MON-YYYY') is basically an equivalent to TO_DATE('31-JUL-2009 00:00:00', 'DD-MON-YYYY HH24:MI:SS'), all the transactions happened after 0:00am on 31 July 2009 would be missed out.

That kind of mistake is pretty common. Sometimes it’s hard to tell by just looking at the data whether a particular DATE column can have date portion. Even if all the values in there are rounded to 0:00 hours, that doesn’t mean that a different time value can’t appear there in the future. The data dictionary can’t help us here either – DATE type is always the same whether it contains time or not. (By the way, Oracle recommends using TIMESTAMP type for new projects, but that is a whole different story.)

If you are working with an existing table and you are not sure, you can use a fool-proof method like this:

SELECT txn_no,
       txn_amount
FROM   bank_transactions
WHERE  txn_date BETWEEN To_date('01-JAN-2009','DD-MON-YYYY')
AND    To_date('31-JUL-2009','DD-MON-YYYY') + 1  1/24/3600

+1 – 1/24/3600 here means “Plus 1 day minus 1 second”. That is because “1” in DATE type means “1 day”, “1/24” - 1 hour, and there are 3600 seconds in an hour.

The above expression will retrieve all the transactions from “01 January 2009 0:00am” to “31 July 2009 0:00am plus 1 day minus 1 second”, i.e. to “31 July 2009 23:59pm”.

If you are charged with designing an application and need to create a table with a DATE column, it is worth to keep yourself and others from future confusions by a simple trick: name columns that only contain date portions as DATE and add TIME to the name of the columns that you know will contain time components. In our case it would be prudent to call the date/time column TXN_DATE_TIME.

The second issue I’d like to discuss is much more subtle, but can do even more damage.

Imagine that you are charged with developing a report that returns all the transaction for the previous month. It looks like a job for SYSDATE! You fetch your trusty keyboard and after a few minutes of typing you come up with something like this:

SELECT txn_no,
       txn_amount
FROM   bank_transactions
WHERE  txn_date BETWEEN Last_day(Add_months(Trunc(SYSDATE),-2)) + 1
AND    Last_day(Add_months(Trunc(SYSDATE),-1))

You create a few lines in BANK_TRANSACTIONS table, run a few unit tests to make sure your code works and check it into the source control. Job done! You congratulate yourself on the productive work and spend the rest of the day reading your friends’ blogs and dreaming about your next vacation. And the next day you move on to another task and get as busy as ever.

After some time, which may be a few days or months, depending on the pace of the project, the code you wrote gets migrated into the UAT environment. And a task force of a few testers and end users is assigned to test the report you wrote. And as it often happens in UAT, they are going to test in on real data they extracted from the production system – that is, the last year’s data.

Got it? Last year’s.

The final stages of testing, such as UAT, have to prove that the system does what it is expected to do in conditions that resemble the production as closely as possible. And the best way to do that is to test it on the retrospective production data – the data that is proven. That makes it possible to compare the outcome to the actual production system, and thus, prove or disprove that the new system works.

That sounds reasonable. But one of the implications for you is that BANK_TRANSACTIONS table is not going to contain previous month’s transactions. Hence, your report will be blank. You can’t rewind back time because you hard-coded SYSDATE, which has only one meaning – “right now”. Test failed.

If you have known that when you wrote it, you wouldn’t have used the SYSDATE. You would use a parameter, something like v_run_date, which you could set to whatever date you wanted. And that would do. Well, now you know.

Make it beautiful

You only need a single look at Sydney Opera House to recognise that it is a work of art. Any masterpiece is like that – you don’t need to do a throughout examination of Mona Lisa’s smile to realise its beauty – you see it instantly. Perfection needs no explanation, it works on subconscious level.

The same applies to the software engineering too. Great code always looks good. It is always carefully formatted, indented and commented. By just looking at it you can tell that it is a work of art. Such code will always work, do what it is supposed to do and have a very few bugs. Because whoever wrote that code cared a lot about it. And you can safely assume that if anyone has put a lot of effort into making the code looking good, he has put at least as much effort into designing and debugging it.

What is even more important, carefully carved code is easier to maintain. In modern software projects any single procedure gets tweaked and rewritten tens of times. If you are a programmer, good chances that even in a project that you work on right now you inherited some code that was written years ago, maybe from people who long left the company. And when you finish with it, it will not be the end of the story – the code will be passed to QA and finally to the production support. And then the cycle will start again. Hence, whatever you program, it’s not just about you. You don’t know how many people will be looking into your code trying to make sense of it. And you can help them immensely by making it is easy to read and understand now. Whether you will be remembered as a good programmer or cursed depends on it.

So, you made an effort to write the code that works. Now make an extra step – make it beautiful.

Date conversion in Oracle part 2

It’s a follow-up to the previous post.

As it turned out, implicit date conversions may also prevent Oracle from doing the partition pruning. For example, if you have a table INVOICES with a range partition on INVOICE_DATE field, then expression

SELECT
...
WHERE invoice_date >= '01-MAR-09'
  AND invoice_date <  '02-MAR-09'

will not perform the partition pruning, whereas

SELECT
...
WHERE invoice_date >= TO_DATE('01/03/2009', 'DD/MM/YYYY')
 AND invoice_date <  TO_DATE('02/03/2009', 'DD/MM/YYYY')

will.

Because the efficiency of partition pruning is usually why partitioning is used in the first place, the choice is obvious.

But after all, I’d use

SELECT
...
WHERE invoice_date BETWEEN TO_DATE( '01/03/2009', 'DD/MM/YYYY')
AND TO_DATE( '01/03/2009', 'DD/MM/YYYY') + 1 - 1/24/3600

since BETWEEN operation is specifically tailored for such situations. “1/24/3600” here represents 1 second, and the whole statement should be read as “From 01 March 2009 0:00am to 01 March 2009 11:59pm”.

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