The output of my sql select statement is a list of Customer ID in column A.
In column B i want to randomly assign each cust ID either bucket A or B to perform A/B testing.
How do I use rand to generate a or b in a new column in my select statement
You could use a solution like the one here:
Generating Random Number In Each Row In Oracle Query which was addressing issues with the use of dbms_random.value.
If you want to get a value that is either 0 or 1, then you can do that like this, assuming your customers are coming from a table named customers:
SELECT customer_id, FLOOR(dbms_random.value+0.5)
FROM customers
The random value is between 0 and 1 (including 0 and not including 1) so adding 0.5 and using floor means you will get half 0's and half 1's.
Notes on using dbms_random in Oracle 11g are available in their manual here: https://docs.oracle.com/cd/B28359_01/appdev.111/b28419/d_random.htm
You may need to initialize the seed to guarantee or improve randomness.
And if 0 and 1 are not appropriate, you could then wrap the FLOOR in a CASE or DECODE to turn the numbers 0 and 1 into the letters A and B.
I have a table with 3 columns a, b and c. I want to know how to update the value of third column with concatenate of two other columns in each row.
before update
A B c
-------------
1 4
2 5
3 6
after update
A B c
-------------
1 4 1_4
2 5 2_5
3 6 3_6
How can I do this in oracle?
Use the concatentation operator ||:
update mytable set
c = a || '_' || b
Or better, to avoid having to rerun this whenever rows are inserted or updated:
create view myview as
select *, a || '_' || b as c
from mytable
Firstly, you are violating the rules of normalization. You must re-think about the design. If you have the values in the table columns, then to get a computed value, all you need is a select statement to fetch the result the way you want. Storing computed values is generally a bad idea and considered a bad design.
Anyway,
Since you are on 11g, If you really want to have a computed column, then I would suggest a VIRTUAL COLUMN than manually updating the column. There is a lot of overhead involved with an UPDATE statement. Using a virtual column would reduce a lot of the overhead. Also, you would completely get rid of the manual effort and those lines of code to do the update. Oracle does the job for you.
Of course, you will use the same condition of concatenation in the virtual column clause.
Something like,
Column_c varchar2(50) GENERATED ALWAYS AS (column_a||'_'||column_b) VIRTUAL
Note : There are certain restrictions on its use. So please refer the documentation before implementing it. However, for the simple use case provided by OP, a virtual column is a straight fit.
Update I did a small test. There were few observations. Please read this question for a better understanding about how to implement my suggestion.
I am creating some record which have id, ts ... So firstly I call select to get ts and id:
select SEQ_table.nextval, CURRENT_TIMESTAMP from dual
and then I call insert
insert into table ...id, ts ...
this works good in 99 % but sometimes when there is a big load the order of record is bad because I need record.id < (record+1).id and record.ts < (record+1).ts but this conditional is met. How I can solve this problem ? I am using oracle database.
You should not use the result of a sequence for ordering. This might look strange but think about how sequences are cached and think about RAC. Every instance has it's own sequence cache .... For performance you need big caches. sequences had better be called random unique key generators that happen to work sequenctially most of the time.
The timestamp format has a time resolution upto microsecond level. When hardware becomes quicker and load increases it could be that you get multiple rows at the same time. There is not much you can do about that, until oracle takes the resolution a step farther again.
Use an INSERT trigger to populate the id and ts columns.
create table sotest
(
id number,
ts timestamp
);
create sequence soseq;
CREATE OR REPLACE TRIGGER SOTEST_BI_TRIG BEFORE
INSERT ON SOTEST REFERENCING NEW AS NEW FOR EACH ROW
BEGIN
:new.id := soseq.nextval;
:new.ts := CURRENT_TIMESTAMP;
END;
/
PHIL#PHILL11G2 > insert into sotest values (NULL,NULL);
1 row created.
PHIL#PHILL11G2 > select * from sotest;
ID TS
---------- ----------------------------------
1 11-MAY-12 13.29.33.771515
PHIL#PHILL11G2 >
You should also pay attention to the other answer provided. Is id meant to be a meaningless primary key (it usually is in apps - it's just a key to join on)?
I have the following Oracle table:
GAMES
id_game int
id_user int
dt_game DATE
Before creating a new record I must check that the same user has not inserted a game more than N times a day.
I'm actually selecting the number of games played today in this way:
select count(1) as num from GAMES
where id_user=ID_USER and
to_char(dt_game,'DD/MM/YYYY')=to_char(sysdate,'DD/MM/YYYY')
I don't like it very much. Is there a better way to do it?
Thanks in advance.
The date conversions are a bit pointless and prevent any index on that column being used; you could simplify that bit with dt_game > trunc(sysdate).
There is a problem (related to concurrency) with checking the table and counting the rows before inserting a new row.
Let us say the user currently has played 9 games and you want to limit the rows to 10..
if you open two different sessions, then the both of them will see the currently uncommitted value of 9 games and hence both the games are allowed, By the time the transaction is complete, the user would have played 11 games..which is against the rule that you are trying to enforce.
You can use a unique constraint to ensure that a fixed maximum number of records may be inserted for a particular day. For example, if N=5:
CREATE TABLE GAMES (
id_game NUMBER NOT NULL,
id_user NUMBER NOT NULL,
dt_game DATE NOT NULL,
user_game_count NUMBER NOT NULL,
CONSTRAINT max_games_per_day
CHECK (user_game_count BETWEEN 1 AND 5),
CONSTRAINT dt_game_notime
CHECK (dt_game = TRUNC(dt_game)),
CONSTRAINT games_unique
UNIQUE (id_user, dt_game, user_game_count),
);
The max_games_per_day constraint means that it can only take one of 5 different values. The dt_game_notime constraint means that we won't get various time values. The unique constraint, finally, ensures that for any particular date and user, they can only insert up to 5 rows.
In Oracle you can create a function based index on id_user, to_char(dt_game,'DD/MM/YYYY') to improve lookup performance.
I have table with "varchar2" as primary key.
It has about 1 000 000 Transactions per day.
My app wakes up every 5 minute to generate text file by querying only new record.
It will remember last point and process only new records.
Do you have idea how to query with good performance?
I am able to add new column if necessary.
What do you think this process should do by?
plsql?
java?
Everyone here is really really close. However:
Scott Bailey's wrong about using a bitmap index if the table's under any sort of continuous DML load. That's exactly the wrong time to use a bitmap index.
Everyone else's answer about the PROCESSED CHAR(1) check in ('Y','N')column is right, but missing how to index it; you should use a function-based index like this:
CREATE INDEX MY_UNPROCESSED_ROWS_IDX ON MY_TABLE
(CASE WHEN PROCESSED_FLAG = 'N' THEN 'N' ELSE NULL END);
You'd then query it using the same expression:
SELECT * FROM MY_TABLE
WHERE (CASE WHEN PROCESSED_FLAG = 'N' THEN 'N' ELSE NULL END) = 'N';
The reason to use the function-based index is that Oracle doesn't write index entries for entirely NULL values being indexed, so the function-based index above will only contain the rows with PROCESSED_FLAG = 'N'. As you update your rows to PROCESSED_FLAG = 'Y', they'll "fall out" of the index.
Well, if you can add a new column, you could create a Processed column, which will indicate processed records, and create an index on this column for performance.
Then the query should only be for those rows that have been newly added, and not processed.
This should be easily done using sql queries.
Ah, I really hate to add another answer when the others have come so close to nailing it. But
As Ponies points out, Oracle does have a hidden column (ORA_ROWSCN - System Change Number) that can pinpoint when each row was modified. Unfortunately, the default is that it gets the information from the block instead of storing it with each row and changing that behavior will require you to rebuild a really large table. So while this answer is good for quieting the SQL Server fella, I'd not recommend it.
Astander is right there but needs a few caveats. Add a new column needs_processed CHAR(1) DEFAULT 'Y' and add a BITMAP index. For low cardinality columns ('Y'/'N') the bitmap index will be faster. Once you have the rest is pretty easy. But you've got to be careful not select the new rows, process them and mark them as processed in one step. Otherwise, rows could be inserted while you are processing that will get marked processed even though they have not been.
The easiest way would be to use pl/sql to open a cursor that selects unprocessed rows, processes them and then updates the row as processed. If you have an aversion to walking cursors, you could collect the pk's or rowids into a nested table, process them and then update using the nested table.
In MS SQL Server world where I work, we have a 'version' column of type 'timestamp' on our tables.
So, to answer #1, I would add a new column.
To answer #2, I would do it in plsql for performance.
Mark
"astander" pretty much did the work for you. You need to ALTER your table to add one more column (lets say PROCESSED)..
You can also consider creating an INDEX on the PROCESSED ( a bitmap index may be of some advantage, as the possible value can be only 'y' and 'n', but test it out ) so that when you query it will use INDEX.
Also if sure, you query only for every 5 mins, check whether you can add another column with TIMESTAMP type and partition the table with it. ( not sure, check out again ).
I would also think about writing job or some thing and write using UTL_FILE and show it front end if it can be.
If performance is really a problem and you want to create your file asynchronously, you might want to use Oracle Streams, which will actually get modification data from your redo log withou affecting performance of the main database. You may not even need a separate job, as you can configure Oracle Streams to do Asynchronous replication of the changes, through which you can trigger the file creation.
Why not create an extra table that holds two columns. The ID column and a processed flag column. Have an insert trigger on the original table place it's ID in this new table. Your logging process can than select records from this new table and mark them as processed. Finally delete the processed records from this table.
I'm pretty much in agreement with Adam's answer. But I'd want to do some serious testing compared to an alternative.
The issue I see is that you need to not only select the rows, but also do an update of those rows. While that should be pretty fast, I'd like to avoid the update. And avoid having any large transactions hanging around (see below).
The alternative would be to add CREATE_DATE date default sysdate. Index that. And then select records where create_date >= (start date/time of your previous select).
But I don't have enough data on the relative costs of setting a sysdate as default vs. setting a value of Y, updating the function based vs. date index, and doing a range select on the date vs. a specific select on a single value for the Y. You'll probably want to preserve stats or hint the query to use the index on the Y/N column, and definitely want to use a hint on a date column -- the stats on the date column will almost certainly be old.
If data are also being added to the table continuously, including during the period when your query is running, you need to watch out for transaction control. After all, you don't want to read 100,000 records that have the flag = Y, then do your update on 120,000, including the 20,000 that arrived when you query was running.
In the flag case, there are two easy ways: SET TRANSACTION before your select and commit after your update, or start by doing an update from Y to Q, then do your select for those that are Q, and then update to N. Oracle's read consistency is wonderful but needs to be handled with care.
For the date column version, if you don't mind a risk of processing a few rows more than once, just update your table that has the last processed date/time immediately before you do your select.
If there's not much information in the table, consider making it Index Organized.
What about using Materialized view logs? You have a lot of options to play with:
SQL> create table test (id_test number primary key, dummy varchar2(1000));
Table created
SQL> create materialized view log on test;
Materialized view log created
SQL> insert into test values (1, 'hello');
1 row inserted
SQL> insert into test values (2, 'bye');
1 row inserted
SQL> select * from mlog$_test;
ID_TEST SNAPTIME$$ DMLTYPE$$ OLD_NEW$$ CHANGE_VECTOR$$
---------- ----------- --------- --------- ---------------------
1 01/01/4000 I N FE
2 01/01/4000 I N FE
SQL> delete from mlog$_test where id_test in (1,2);
2 rows deleted
SQL> insert into test values (3, 'hello');
1 row inserted
SQL> insert into test values (4, 'bye');
1 row inserted
SQL> select * from mlog$_test;
ID_TEST SNAPTIME$$ DMLTYPE$$ OLD_NEW$$ CHANGE_VECTOR$$
---------- ----------- --------- --------- ---------------
3 01/01/4000 I N FE
4 01/01/4000 I N FE
I think this solution should work..
What you need to do following steps
For the first run, you will have to copy all records. In first run you need to execute following query
insert into new_table(max_rowid) as (Select max(rowid) from yourtable);
Now next time when you want to get only newly inserted values, you can do it by executing follwing command
Select * from yourtable where rowid > (select max_rowid from new_table);
Once you are done with processing above query, simply truncate new_table and insert max(rowid) from yourtable
I think this should work and would be fastest solution;