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Oracle Delete Taking a Long Time

Tim — Sun, 17 Jan 2010 02:16:15 +0000

It may look like your delete statement is hung, but ff your table has foreign keys in another table without indexes on that column Oracle will full table scan that other table for every row that you’re deleting. Hopefully this saves someone some time.

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Luhn Digit Generation Oracle PL/SQL Implementation

Tim — Tue, 06 Oct 2009 11:27:06 +0000

Here's how to generate a Luhn Mod-10 Check Digit. It's implemented as a function in oracle PL/SQL. I spent 2 hours looking for this online, then decided to implement it myself since it's not that hard. Anyway, I thought I'd post it here for the next person trolling google for it. This is useful for generating credit card numbers or any other place you need a check digit.

This algorithm works by doing the same thing as the validation algorithm, but shifted over one character (the equivalent of adding a zero to a number, generating the sum & subtracting 10).

*EDIT - there was a bug in this code, i just fixed it on 12/4/2009, the old code returned 10 instead of zero. It also was only working on even numbers, sorry about that!

CREATE OR REPLACE FUNCTION generate_luhn (togen IN varchar)
RETURN number
IS
curval NUMBER := 0;
total NUMBER := 0;
everyother NUMBER := 1;

BEGIN
IF(
togen IS NULL
) then
RETURN 0;
end IF;

FOR i IN reverse 1 .. LENGTH(togen) loop
curval:=SUBSTR(togen, i, 1);
IF everyother = 1 then
everyother := 0;
curval := curval * 2;
IF(curval> 9) then
curval:=curval-9;
end IF;
else
everyother := 1;
end IF;

total := total + curval;
end loop;
IF MOD(total, 10) = 0 THEN
RETURN 0;
ELSE
RETURN 10 - MOD(total, 10);
END IF;
END generate_luhn;
/

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Postgres select replace with regular expression

Tim — Wed, 02 Jan 2008 21:22:50 +0000

Postgres seriously kicks ass, here's a quick way to do a substring based on a regular expression. A Co-worker wanted to get the first word out of a column that contained a sentence. If we were using something lame like oracle we'd have to do gymnastics to do that.

It's a one-liner in postgres.

SELECT substring('this is a sentence' FROM '^(.+?)\\s.+$')

So we're selecting out the group that is a series of characters at the start of a line (column) before a space. Epic win.

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Randomizing Query Results in Oracle

Tim — Wed, 20 Jun 2007 17:45:14 +0000

I was doing some work this afternoon, and I needed some sample id's from a database query, however all the data ended up being related to the same person because of the ordering of the results. There must be a better way, i want random results in my resultset.

Here's how (in oracle), order by a random value using dbms_random.value()

SELECT * FROM OURTABLE ORDER BY dbms_random.value()

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Accessing main query aliases from subquery

Tim — Fri, 16 Mar 2007 13:42:53 +0000

I am stupid, I never fully understood Oracle Subqueries. Let me give you an example, I had a really big view where I wanted to access stats of child tables based on a parent table.

For example, let's call that table checks, I wanted to know how many responses were linked to each check. We have a a one to many relationship between checks and responses.

So I wrote my original query something like this:

SELECT c.check_id,cr.count_of_responses
FROM c.checks
WHERE
LEFT JOIN (SELECT count(1),r.check_id AS count_of_responses FROM responses r GROUP BY r.check_id) cr
ON cr.check_id = c.check_id

This works BUT, it produces a full tablescan on the child table unless you specify a check_id in the main query. This aint that useful if you want to say query on the count_of_responses for all checks. What if we could access the main query variables in the subquery? Then the database wouldn't have to full tablescan and join it all together.

Well you can if you do your subquery in the SELECT clause, the only catch is you can only select one thing. The good thing is it doesn't do full tablescans, it will use your foriegn key indexes!

So our rewritten query would look like this:

SELECT c.check_id,(SELECT count(1) FROM responses r WHERE r.check_id = c.check_id)
FROM c.checks

Golden, now for each row it just joins using the index pulling back a single row from responses!

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Formatting Numbers in Oracle and Postgres

Tim — Mon, 09 Oct 2006 12:29:06 +0000

When dealing with large numbers in oracle sometimes it can be hard to read the results from a query with no formatting in the results. I know I have misread numbers from a query many times. Well, I finally got around to looking up how to format numbers in oracle. And who knew -- TO_CHAR does it for us.

Example In Oracle:

SELECT to_char(5689731,'$9,999,999.00') AS mysalary FROM DUAL

Jason says this also works in Postgres!
Example in Postgres:

SELECT to_char(5689731, '$9,999,999.00') AS mysalary

returns

$5,689,731.00

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