Tuesday, May 24, 2011

Cannot restart LDAP server on Ubuntu

I did some changes to the ldif file yesterday, and cannot start my LDAP server any more today. I tried different ways, but all unfortunately failed.

Actually the way to workaround is quite easy, that you simply remove the folder slapd.d/ under /etc/ldap, and reinstall the ldap again, ldap will work again.

Intellij IDEA 10.5 stuck in the loading process

Intellij IDEA is a really smart IDA that attracts more and more developers to its community. However, a lot of bugs are still hanging there which annoy people too much.

One of the bugs is when you start IDEA with a pretty large project, there is a highly chance that your loading process will stuck there forever. This has been identified as a bug in IDEA-67401, and hasn't been resolved yet now.

According to that issue page, there is a workaround to load the project. That is to disable the 'Tip of the Day' and 'Productivity Guide' prompt when start a new project or open IDEA.

Besides, there is another possibility that you cannot even get in to adjust your setting before it get stuck. If that is the case, just remember to be very quick when you open IDEA, as soon as the loading project prompt pop up, cancel the loading, as well as all the next loading processes. Once the loading started, you will not be able to cancel it again, and have to kill the IDE and restart again.

Monday, May 16, 2011

Oracle: To convert a row into a column (CROSS-JOIN)

This afternoon, while I am doing some Jasper report stuff, I am puzzled by a SQL query that which takes quite a long time to execute. As the time goes, Jasper will easily get a timeout and a blank screen is waiting for me after retrieving data for about 120 seconds.

As it is not appropriate to use the original data table as the example to describe here, I tried to make up a similar scenario which should explain the cause and the solution sufficiently.

Given a table A, with five columns -- Primary Key (pk), Statistics 1A (s1a), Statistics 1B (s1b), Statistics 2A(s2a), Statistics 2B(s2b), we need to select the data out into a form that looks like -- Primary Key (pk), Statistics A(s1a or s2a), Statistics B(s2b or s2b). Essentially, it is trying to convert a row into one column but two rows.  Someone may prefer to call it pivot query by the way.

Initially, naively I think the most easiest and intuitive way to do it is by two union queries. That is
select pk, s1a, s1b from table where *** union select pk, s2a, s2b from table where ***.
When the data set is small, and it only depends on the single table, it is fine. However, in my case, the data set is huge, and the union will not be two but eight, and more importantly every union section will consists of another 6 inner join tables. That's the reason why Jasper cannot retrieve what it needs on time.

In this link, as suggested by Scott, introduced a very smart way to handle this issue. What it uses is called: cross join. So basically, we use the existing table to cross join with the different type/criteria to return the expected result.

      when ite = 's1' then s1a
      when ite = 's2' then s2a
   end as sa
      when ite = 's1' then s1b
      when ite = 's2' then s2b
   end as sb
       select pivoter.ite,
           s1a, s1b, s2a, s2b
           cross join (
               select 's1' as ite from dual
               union all
               select 's2' as ite from dual
           ) pivotel

By using the above code, the problem will be solved by the cross join union columns.

Sunday, May 15, 2011

Miscellaneous Points for Oracle Join Query

As a software developer, you must have the experience to deal with all sorts of join queries. And I believe everyone has once or still now struggle with all these different terms about inner, outer, left, right, and etc. As I have spent roughly two hours these morning, to summarize some of the points that I am easily to forgotten and got wrong, it's a good chance to write it done in case later I need to test my memory.

1) What is the difference of join...on... grammar and little (+) sign?

Oracle used to only support (+) in old days, which is also kind of created by Oracle. Later, as the ANSI formalized the standard for join grammar, Oracle adopted both for join queries. As a result, it won't be a surprise if you see both two styles in your project which comes from different developers.

2) Any difference between inner join and join?


3) Why do we need outer join?

Generally, inner join will return result sets that have a match in both Table A and B. However in reality sometimes, we also need results to be returned even the matching is null. That's the place where outer join shows its ability. There are plenty of tutorials available to discuss about the outer join. What I want to mentioning here is the null value will only applied to the join's destination table, but not the starting table.

4) Any more differences?

Yes. Actually there are two more I want to emphasize:

1. ANSI style supports outer join, which you can google what does it mean. But traditional Oracle doesn't directly support it. ( By saying it directly, I mean there always exists workarounds, and various workarounds)
2. One of the most important feature to differentiate the two is the ANSI style separate the join condition and query filter criteria, which is much more nice tidy and clean. (Is it, at least I think so). Meanwhile, you will also be able to avoid a lot of caveats that may come with you when you go for the traditional style. See Common errors seen when using OUTER-JOIN.