I am trying to implement a table with revision history in Postgresql as follows:
table has a multi-column primary key or unique constraint on columns id and rev (both numeric)
to create a new entry, insert data, have id auto-generated and rev set to 0
to update an existing entry, insert a new row with the previous id and rev set to -1, then increment the rev on all entries with that id by 1
to get the latest version, select by id and rev = 0
The problem that I am facing is the update after the insert; unsurprisingly, Postgresql sometimes raises a "duplicate key" error when rows are updated in the wrong order.
Since there is no ORDER BY available on UPDATE, I searched for other solutions and noticed that the updates pass without errors if the starting point is the result set of an subquery:
UPDATE test
SET rev = test.rev + 1
FROM (
SELECT rev
FROM test
WHERE id = 99
ORDER BY rev DESC
) AS prev
WHERE test.rev = prev.rev
The descending order ensures that the greater values get incremented first, so that the lower values do not violate the unique constraint when they get updated.
One catch though; I can't derive from the documentation if this is working due to some implementation detail (which might change without notice in the future) or indeed guaranteed by the language specification - can someone explain?
I was also wondering whether it is performance-wise better to have the rev column in the index (as described above, which leads to at least a partial index rebuild on every update, but maybe also to faster reads) or to define a (non-unique) index on id only and ignore the performance impact that could be caused by an (initially) larger query set. (I am expecting a rather low revision count per unique id on average, maybe 5.)
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...
My tables have a RowVersion column called LastChanged.
ID | LastChanged | Foo |
I am developing some sync related functionality. I will be selecting all records from the table between a min and max RowVersion. The initial sync won't have a Min Row Version so I will be including all rows upto MIN_ACTIVE_ROWVERSION().
Subsequent syncs will have a min RowVersion - typically it will be the MIN_ACTIVE_ROWVERSION() from the previous sync.
Selecting rows that are between the Min and Max RowVersion like this is easy. However I would also like to determine, which of those rows, are Inserts and which rows are Updates. The easiest way for me to do this, is to add another column:
ID | LastChanged (RowVersion) | CreationRowVersion (Binary(8)) | Foo |
For CreationRowVersion - The idea is to capture the RowVersion value on insert. That value will then never change for the row. So I would like to default CreationRowVersion to the same value as RowVersion when the row is initially Inserted.
With this in place, I should then be able to determine which rows have been created, and which rows have been updated since the last sync (i.e between min and max RowVersions) - because for created rows, I can look at rows that have a CreationRowVersion that fall within the min and max row version range. For Updated Rows, I can look at rows that have a LastChanged that fall within min and max row version range - but I can also exclude rows from being detected as "Updates" if their CreationRowVersion also falls between min and max RowVersions as then I know they are actually already included as Inserts.
So now that the background is out of the way, it brings me to the crux of my question. What is the most efficient way to default CreationRowVersion to the RowVersion on Insert? Can this be done with a default constrain on the column, or does it have to be done via a trigger? I'd like this column to be a Binary(8) as this matches the datatype of RowVersion.
Thanks
Try using the MIN_ACTIVE_ROWVERSION() function as the default value for your CreationRowVersion BINARY(8) column.
CREATE TABLE dbo.RowVerTest (
ID INT IDENTITY,
LastChanged ROWVERSION,
CreationRowVersion BINARY(8)
CONSTRAINT DF_RowVerTest_CreationRowVersion DEFAULT(MIN_ACTIVE_ROWVERSION()),
Foo VARCHAR(256)
)
GO
INSERT INTO dbo.RowVerTest (Foo) VALUES ('Hello');
GO
--[LastChanged] and [CreationRowVersion] should be equal.
SELECT * FROM dbo.RowVerTest;
GO
UPDATE dbo.RowVerTest SET Foo = 'World' WHERE ID = 1;
GO
--[LastChanged] should be incremented, while [CreationRowVersion]
--should retain its original value from the insert.
SELECT * FROM dbo.RowVerTest;
GO
CAUTION: in my testing, the above only works when rows are inserted one at a time. The code for the scenario below does not appear to work for your use case:
--Insert multiple records with a single INSERT statement.
INSERT INTO dbo.RowVerTest (Foo)
SELECT TOP(5) name FROM sys.objects;
--All the new rows have the same value for [CreationRowVersion] :{
SELECT * FROM dbo.RowVerTest;
There is an existing question about referencing columns in a default statement. You can't do it, but there are other suggestions to look at, including an AFTER INSERT trigger.
You may want to take a look at this question on RowVersion and Performance.
Hello I have a simple table like that:
+------------+------------+----------------------+----------------+
|id (serial) | date(date) | customer_fk(integer) | value(integer) |
+------------+------------+----------------------+----------------+
I want to use every row like a daily accumulator, if a customer value arrives
and if doesn't exist a record for that customer and date, then create a new row for that customer and date, but if exist only increment the value.
I don't know how implement something like that, I only know how increment a value using SET, but more logic is required here. Thanks in advance.
I'm using version 9.4
It sounds like what you are wanting to do is an UPSERT.
http://www.postgresql.org/docs/devel/static/sql-insert.html
In this type of query, you update the record if it exists or you create a new one if it does not. The key in your table would consist of customer_fk and date.
This would be a normal insert, but with ON CONFLICT DO UPDATE SET value = value + 1.
NOTE: This only works as of Postgres 9.5. It is not possible in previous versions. For versions prior to 9.1, the only solution is two steps. For 9.1 or later, a CTE may be used as well.
For earlier versions of Postgres, you will need to perform an UPDATE first with customer_fk and date in the WHERE clause. From there, check to see if the number of affected rows is 0. If it is, then do the INSERT. The only problem with this is there is a chance of a race condition if this operation happens twice at nearly the same time (common in a web environment) since the INSERT has a chance of failing for one of them and your count will always have a chance of being slightly off.
If you are using Postgres 9.1 or above, you can use an updatable CTE as cleverly pointed out here: Insert, on duplicate update in PostgreSQL?
This solution is less likely to result in a race condition since it's executed in one step.
WITH new_values (date::date, customer_fk::integer, value::integer) AS (
VALUES
(today, 24, 1)
),
upsert AS (
UPDATE mytable m
SET value = value + 1
FROM new_values nv
WHERE m.date = nv.date AND m.customer_fk = nv.customer_fk
RETURNING m.*
)
INSERT INTO mytable (date, customer_fk, value)
SELECT date, customer_fk, value
FROM new_values
WHERE NOT EXISTS (SELECT 1
FROM upsert up
WHERE up.date = new_values.date
AND up.customer_fk = new_values.customer_fk)
This contains two CTE tables. One contains the data you are inserting (new_values) and the other contains the results of an UPDATE query using those values (upsert). The last part uses these two tables to check if the records in new_values are not present in upsert, which would mean the UPDATE failed, and performs an INSERT to create the record instead.
As a side note, if you were doing this in another SQL engine that conforms to the standard, you would use a MERGE query instead. [ https://en.wikipedia.org/wiki/Merge_(SQL) ]
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 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