Friday, November 6, 2009

Don't mess with LIKE

Oh boy, It looks like it's time to change the title of this blog to "A hundred ways you can screw up with Oracle". I may need a new domain name, something flashy. would be fine. Let me see, it is currently available. I wonder how fast cyber-squatters will jump in and snatch it now when I pronounced it. Then they will blackmail me demanding a outrageous ransom or else... (insert evil face here)

On second thought, no. I don't want to turn this blog into an Oracle-specific. There are already enough Oracle blogs out there. In fact, I think there are more of them than Oracle professionals who would read them. I don't want to bring yet another one to the world just for the sake of it. Heck, it's my place and I'm going to write about whatever I want. Hence the title, Random Thoughts.

Ok, kids, take your places. Today's lesson is (surprise, surprise!) about Oracle. We already discussed a few ways we can screw up with dates. Today we will talk about numbers. On the surface numbers look like pretty innocent data type. But once you dive a little deeper... Beware! Fearful creatures lurk beneath. And if you are not careful, they will snatch you in no time.

Take a look at this question by James Collins.
James had a problem, the following query was slow:

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:

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:

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));
  2. 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.

  3. Create a separate column ID_CHAR of type VARCHAR2 and fill it with TO_CHAR(id). Index it and use instead of ID in your WHERE condition.
  4. 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.

Wednesday, November 4, 2009

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,
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,
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,
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.

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