I have a table in my database and I want for each row in my table to have an unique id and to have the rows named sequently.
For example: I have 10 rows, each has an id - starting from 0, ending at 9. When I remove a row from a table, lets say - row number 5, there occurs a "hole". And afterwards I add more data, but the "hole" is still there.
It is important for me to know exact number of rows and to have at every row data in order to access my table arbitrarily.
There is a way in sqlite to do it? Or do I have to manually manage removing and adding of data?
Thank you in advance,
Ilya.
It may be worth considering whether you really want to do this. Primary keys usually should not change through the lifetime of the row, and you can always find the total number of rows by running:
SELECT COUNT(*) FROM table_name;
That said, the following trigger should "roll down" every ID number whenever a delete creates a hole:
CREATE TRIGGER sequentialize_ids AFTER DELETE ON table_name FOR EACH ROW
BEGIN
UPDATE table_name SET id=id-1 WHERE id > OLD.id;
END;
I tested this on a sample database and it appears to work as advertised. If you have the following table:
id name
1 First
2 Second
3 Third
4 Fourth
And delete where id=2, afterwards the table will be:
id name
1 First
2 Third
3 Fourth
This trigger can take a long time and has very poor scaling properties (it takes longer for each row you delete and each remaining row in the table). On my computer, deleting 15 rows at the beginning of a 1000 row table took 0.26 seconds, but this will certainly be longer on an iPhone.
I strongly suggest that you re-think your design. In my opinion your asking yourself for troubles in the future (e.g. if you create another table and want to have some relations between the tables).
If you want to know the number of rows just use:
SELECT count(*) FROM table_name;
If you want to access rows in the order of id, just define this field using PRIMARY KEY constraint:
CREATE TABLE test (
id INTEGER PRIMARY KEY,
...
);
and get rows using ORDER BY clause with ASC or DESC:
SELECT * FROM table_name ORDER BY id ASC;
Sqlite creates an index for the primary key field, so this query is fast.
I think that you would be interested in reading about LIMIT and OFFSET clauses.
The best source of information is the SQLite documentation.
If you don't want to take Stephen Jennings's very clever but performance-killing approach, just query a little differently. Instead of:
SELECT * FROM mytable WHERE id = ?
Do:
SELECT * FROM mytable ORDER BY id LIMIT 1 OFFSET ?
Note that OFFSET is zero-based, so you may need to subtract 1 from the variable you're indexing in with.
If you want to reclaim deleted row ids the VACUUM command or pragma may be what you seek,
http://www.sqlite.org/faq.html#q12
http://www.sqlite.org/lang_vacuum.html
http://www.sqlite.org/pragma.html#pragma_auto_vacuum
Related
I have a situation where I have multiple (potentially hundreds) threads repeating the same task (using a java scheduled executor, if you are curious). This task entails selecting rows of changes (from a table called change) that have not yet been processed (processed changes are kept track in a m:n join table called process_change_rel that keeps track of the process id, record id and status) processing them, then updating back the status.
My question is, how is the best way to prevent two threads from the same process from selecting the same row? Will the below solution (using for update to lock rows ) work? If not, please suggest a working solution
Create table change(
—id , autogenerated pk
—other fields
)
Create table change_process_rel(
—change id (pk of change table)
—process id (pk of process table)
—status)
Query I would use is listed below
Select * from
change c
where c.id not in(select changeid from change_process_rel with cs) for update
Please let me know if this would work
You have to "lock" a row which you are going to process somehow. Such a "locking" should be concurrent of course with minimum conflicts / errors.
One way is as follows:
Create table change
(
id int not null generated always as identity
, v varchar(10)
) in userspace1;
insert into change (v) values '1', '2', '3';
Create table change_process_rel
(
id int not null
, pid int not null
, status int not null
) in userspace1;
create unique index change_process_rel1 on change_process_rel(id);
Now you should be able to run the same statement from multiple concurrent sessions:
SELECT ID
FROM NEW TABLE
(
insert into change_process_rel (id, pid, status)
select c.id, mon_get_application_handle(), 1
from change c
where not exists (select 1 from change_process_rel r where r.id = c.id)
fetch first 1 row only
with ur
);
Every such a statement inserts 1 or 0 rows into the change_process_rel table, which is used here as a "lock" table. The corresponding ID from change is returned, and you may proceed with processing of the corresponding event in the same transaction.
If the transaction completes successfully, then the row inserted into the change_process_rel table is saved, so, the corresponding id from change may be considered as processed. If the transaction fails, the corresponding "lock" row from change_process_rel disappears, and this row may be processed later by this or another application.
The problem of this method is, that when both tables become large enough, such a sub-select may not work as quick as previously.
Another method is to use Evaluate uncommitted data through lock deferral.
It requires to place the status column into the change table.
Unfortunately, Db2 for LUW doesn't have SKIP LOCKED functionality, which might help with such a sort of algorithms.
If, let's say, status=0 is "not processed", and status<>0 is some processing / processed status, then after setting these DB2_EVALUNCOMMITTED and DB2_SKIP* registry variables and restart the instance, you may "catch" the next ID for processing with the following statement.
SELECT ID
FROM NEW TABLE
(
update
(
select id, status
from change
where status=0
fetch first 1 row only
)
set status=1
);
Once you get it, you may do further processing of this ID in the same transaction as previously.
It's good to create an index for performance:
create index change1 on change(status);
and may be set this table as volatile or collect distribution statistics on this column in addition to regular statistics on table and its indexes periodically.
Note that such a registry variables setting has global effect, and you should keep it in mind...
I use postgresql and I have a database table with more than 5 million records. The structure of the table is as follows:
A lot of records is inserted every day. There are many records with the same reference.
I want to select all records but I do not want duplicates, the records with the same reference.
I tried with query as follows:
SELECT DISTINCT ON (reference) reference_url, reference FROM daily_run_vehicle WHERE handled = False and retries < 5 ORDER BY reference DESC;
It executed, it gives me correct result, but it takes to long to execute.
Is there any better way to do this?
Create Sort keys on columns which yo used in where condition
after large data movement into the table, we need to do "vacuum" command it will refresh all the keys and after that Analyze the table with "Analyze" command. it will help to rebuild the stats of the table.
I have a table with three columns A, B, C, all of type bytea.
There are around 180,000,000 rows in the table. A, B and C all have exactly 20 bytes of data, C sometimes contains NULLs
When creating indexes for all columns with
CREATE INDEX index_A ON transactions USING hash (A);
CREATE INDEX index_B ON transactions USING hash (B);
CREATE INDEX index_C ON transactions USING hash (C);
index_A is created in around 10 minutes, while B and C are taking over 10 hours after which I aborted them. I ran every CREATE INDEX on their own, so no indices were created in parallel. There are also no other queries running in the database.
When running
SELECT * FROM pg_stat_activity;
wait_event_type and wait_event are both NULL, state is active.
Why are the second index creations taking so long, and can I do anything to speed them up?
Ensure the statistics on your table are up-to-date.
Then execute the following query:
SELECT attname, n_distinct, correlation
from pg_stats
where tablename = '<Your table name here>'
Basically, the database will have more work to create indexes when:
The number of distinct values gets higher.
The correlation (= are values in the field physically stored in order) is close to 0.
I suspect you will see field A is different in terms of distinct values and/or a higher correlation than the other 2 fields.
Edit: Basically, creating an index = FULL SCAN of the table and create entries in the index as you progress. With the stats you have shared below that means:
Column A: it was detected as unique
A single scan is enough as the DB knows 1 record = 1 index entry.
Columns B & C : it was detected as having very few distinct values + abs(correlation) is very low.
Each index entry takes an entire FULL SCAN of the table.
Note: the description is simplified to highlight the difference.
Solution 1:
Do not create indexes for B and C.
It might sound stupid but in fact and as explained here, a small correlation means the indexes will probably not be used (an index is useful only when entries are not scattered in all the table blocks).
Solution 2:
Order records on the disk.
The initialization would be something like this:
CREATE TABLE Transactions_order as SELECT * FROM Transactions;
TRUNCATE TABLE Transactions;
INSERT INTO Transactions SELECT * FROM Transactions_order ORDER BY B,C,A;
DROP TABLE Transactions_order;
The tricky part comes next: with insert/update/delete records, you need to keep track of the correlation and ensure it does not drop too much.
If you can't guarantee that, stick to solution 1.
Solution3:
Create partitions and enjoy partition pruning.
There are quite a lot of efforts being made for partitioning recently in postgresql. It could be worth having a look into it.
I have implemented auto increment on my table in oracle using sequence and trigger.
The first two records entered had IDs in a consecutive order, but on the entry of the third record, there was a gap. The ID was 8 for it. On deletion of the record and entery of the new record, the ID became 9.
How do I remove the gap and get the ID of the 3rd record as 3 and in consecutive order for next.
If a row is deleted will the new row start with a new sequence or continue from the deleted sequence?
How do I solve this problem?
Please help
Gaps in a sequence are never reused. You should expect gaps anyway, even without deletions, due to caching, rollbacks, RAC etc. You may not even get the sequences in the order you expect if you're using RAC. An ID, particularly an 'auto-incremented' one, is generally a synthetic primary key and the actual value of the key has no intrinsic meaning, it merely has to be unique.
Tom Kyte has a view on this subject, of course.
If gaps were re-used, what would happen if you added three records which got IDs 1, 2 and 3, and then deleted record 2? Would the next inserted record get ID 2 or 4? Even if you filled in the gap (which would mean serialising and basically ignoring the sequence) then the IDs are not in the order they were inserted, so you still can't discern anything from looking at the IDs. You don't know, from the ID, what order they were inserted.
If you really want seamless numbering for display purposes you can ignore the ID value and use a pseudo-column:
select t.column1, t.column2, ...,
row_number() over (order by t.id) as rn
from your_table t;
If you need to track the order they were inserted then add a timestamp field and set that to sys_timestamp in your trigger as well. You could then generate a seamless number in that order instead:
select t.column1, t.column2, ...,
row_number() over (order by t.inserted) as rn
from your_table t;
I have PostgreSQL table:
Username1 SomeBytes1
Username2 SomeBytes1
Username1 SomeBytes1
Username1 SomeBytes1
I need to get some rows from with name Username1 but from the end of the table. For example i need last to rows with Username1
select from my_table where user = Username1 LIMIT 2
Gives me first 2 rows, but i need last two.
How can i select it?
Thank you.
first and last in a table is very arbitrary. In order to have a good predictable result you should always have an order by clause. And if you have that, then getting the last two rows will become easy.
For instance, if you have a primary key or something like an ID (which is populated by a sequence), then you can do:
select * from my_table where user = 'Username1' order by ID desc limit 2.
desc tells the database to sort the rows in reverse order, which means that last will be first.
Does your table have a primary key ? / Can your table be sorted?
Because the notion of 'first' and 'last' implies some sorting of the tuples. If this is the case, you could sort the data the other way around, so that your 'last' entries are on top. Then you can access them with the statement you tried.
To view tail of a table you may use ctid. It is a temporary physical identifier of a record in PostgreSQL.
SELECT * from my_table
WHERE user = Username1
ORDER BY ctid DESC
LIMIT 2