There are some similar questions on StackOverflow, but they don't seem to exactly match my case. I am trying to bulk insert into a PostgreSQL table with composite unique constraints. I created a temporary table (temptable) without any constraints, and loaded the data (with possible some duplicate values) in it. So far, so good.
Now, I am trying to transfer the data to the actual table (realtable) with unique index. For this, I used an INSERT statement with a subquery:
INSERT INTO realtable
SELECT * FROM temptable WHERE NOT EXISTS (
SELECT 1 FROM realtable WHERE temptable.added_date = realtable.added_date
AND temptable.product_name = realtable.product_name
);
However, I am getting duplicate key errors:
ERROR: duplicate key value violates unique constraint "realtable_added_date_product_name_key"
SQL state: 23505
Detail: Key (added_date, product_name)=(20000103, TEST) already exists.
My question is, shouldn't the WHERE NOT EXISTS clause prevent this from happening? How can I fix it?
The NOT EXISTS clause only prevents rows from temptable conflicting with existing rows from realtable; it will not prevent multiple rows from temptable from conflicting with each other. This is because the SELECT is calculated once based on the initial state of realtable, not re-calculated after each row is inserted.
One solution would be to use a GROUP BY or DISTINCT ON in the SELECT query, to omit duplicates, e.g.
INSERT INTO realtable
SELECT DISTINCT ON (added_date, product_name) *
FROM temptable WHERE NOT EXISTS (
SELECT 1 FROM realtable WHERE temptable.added_date = realtable.added_date
AND temptable.product_name = realtable.product_name
)
ORDER BY ???; -- this ORDER BY will determine which of a set of duplicates is kept by the DISTINCT ON
Related
I want to insert a new record if the record is not present in the table
For that I am using below query in Teradata
INSERT INTO sample(id, name) VALUES('12','rao')
WHERE NOT EXISTS (SELECT id FROM sample WHERE id = '12');
When I execute the above query I am getting below error.
WHERE NOT EXISTS
Failure 3706 Syntax error: expected something between ')' and the 'WHERE' keyword.
Can anyone help with the above issue. It will be very helpful.
You can use INSERT INTO ... SELECT ... as follows:
INSERT INTO sample(id,name)
select '12','rao'
WHERE NOT EXISTS (SELECT id FROM sample WHERE id = '12');
You can also create the primary/unique key on id column to avoid inserting duplicate data in id column.
I would advise writing the query as:
INSERT INTO sample (id, name)
SELECT id, name
FROM (SELECT 12 as id, 'rao' as name) x
WHERE NOT EXISTS (SELECT 1 FROM sample s WHERE s.id = x.id);
This means that you do not need to repeat the constant value -- such repetition can be a cause of errors in queries. Note that I removed the single quotes. id looks like a number so treat it as a number.
The uniqueness of ids is usually handled using a unique constraint or index:
alter table sample add constraint unq_sample_id unique (id);
This makes sure that the database ensures uniqueness. Your approach can fail if two inserts are run at the same time with the same id. An attempt to insert a duplicates returns an error (which the exists can then avoid).
In practice, id columns are usually generated automatically by the database. So the create table statement would look more like:
id integer generated by default as identity
And the insert would look like:
insert into sample (name)
values (name);
If id is the Primary Index of the table you can use MERGE:
merge into sample as tgt
using VALUES('12','rao') as src (id, name)
on src.id = tgt.id
when not matched
then insert (src.id,src.name)
We have a DB for which we need a "selsert" (not upsert) function.
The function should take a text value and return a id column of existing row (SELECT) or insert the value and return id of new row (INSERT).
There are multiple processes that will need to perform this functionality (selsert)
I have been experimenting with pg_advisory_lock and ON CONFLICT clause for INSERT but am still not sure what approach would work best (even when looking at some of the other answers).
So far I have come up with following
WITH
selected AS (
SELECT id FROM test.body_parts WHERE (lower(trim(part))) = lower(trim('finger')) LIMIT 1
),
inserted AS (
INSERT INTO test.body_parts (part)
SELECT trim('finger')
WHERE NOT EXISTS ( SELECT * FROM selected )
-- ON CONFLICT (lower(trim(part))) DO NOTHING -- not sure if this is needed
RETURNING id
)
SELECT id, 'inserted' FROM inserted
UNION
SELECT id, 'selected' FROM selected
Will above query (within function) insure consistency in high
concurrency write workloads?
Are there any other issues I must consider (locking?, etc, etc)
BTW, I can insure that there are no duplicate values of (part) by creating unique index. That is not an issue. What I am after is that SELECT returns existing value if another process does INSERT (I hope I am explaining this right)
Unique index would have following definition
CREATE UNIQUE INDEX body_parts_part_ux
ON test.body_parts
USING btree
(lower(trim(part)));
For a table like this one:
CREATE TABLE Users(
id SERIAL PRIMARY KEY,
name TEXT UNIQUE
);
What would be the correct one-query insert for the following operation:
Given a user name, insert a new record and return the new id. But if the name already exists, just return the id.
I am aware of the new syntax within PostgreSQL 9.5 for ON CONFLICT(column) DO UPDATE/NOTHING, but I can't figure out how, if at all, it can help, given that I need the id to be returned.
It seems that RETURNING id and ON CONFLICT do not belong together.
The UPSERT implementation is hugely complex to be safe against concurrent write access. Take a look at this Postgres Wiki that served as log during initial development. The Postgres hackers decided not to include "excluded" rows in the RETURNING clause for the first release in Postgres 9.5. They might build something in for the next release.
This is the crucial statement in the manual to explain your situation:
The syntax of the RETURNING list is identical to that of the output
list of SELECT. Only rows that were successfully inserted or updated
will be returned. For example, if a row was locked but not updated
because an ON CONFLICT DO UPDATE ... WHERE clause condition was not
satisfied, the row will not be returned.
Bold emphasis mine.
For a single row to insert:
Without concurrent write load on the same table
WITH ins AS (
INSERT INTO users(name)
VALUES ('new_usr_name') -- input value
ON CONFLICT(name) DO NOTHING
RETURNING users.id
)
SELECT id FROM ins
UNION ALL
SELECT id FROM users -- 2nd SELECT never executed if INSERT successful
WHERE name = 'new_usr_name' -- input value a 2nd time
LIMIT 1;
With possible concurrent write load on the table
Consider this instead (for single row INSERT):
Is SELECT or INSERT in a function prone to race conditions?
To insert a set of rows:
How to use RETURNING with ON CONFLICT in PostgreSQL?
How to include excluded rows in RETURNING from INSERT ... ON CONFLICT
All three with very detailed explanation.
For a single row insert and no update:
with i as (
insert into users (name)
select 'the name'
where not exists (
select 1
from users
where name = 'the name'
)
returning id
)
select id
from users
where name = 'the name'
union all
select id from i
The manual about the primary and the with subqueries parts:
The primary query and the WITH queries are all (notionally) executed at the same time
Although that sounds to me "same snapshot" I'm not sure since I don't know what notionally means in that context.
But there is also:
The sub-statements in WITH are executed concurrently with each other and with the main query. Therefore, when using data-modifying statements in WITH, the order in which the specified updates actually happen is unpredictable. All the statements are executed with the same snapshot
If I understand correctly that same snapshot bit prevents a race condition. But again I'm not sure if by all the statements it refers only to the statements in the with subqueries excluding the main query. To avoid any doubt move the select in the previous query to a with subquery:
with s as (
select id
from users
where name = 'the name'
), i as (
insert into users (name)
select 'the name'
where not exists (select 1 from s)
returning id
)
select id from s
union all
select id from i
I am running a python script that inserts a large amount of data into a Postgres database, I use a single query to perform multiple row inserts:
INSERT INTO table (col1,col2) VALUES ('v1','v2'),('v3','v4') ... etc
I was wondering what would happen if it hits a duplicate key for the insert. Will it stop the entire query and throw an exception? Or will it merely ignore the insert of that specific row and move on?
The INSERT will just insert all rows and nothing special will happen, unless you have some kind of constraint disallowing duplicate / overlapping values (PRIMARY KEY, UNIQUE, CHECK or EXCLUDE constraint) - which you did not mention in your question. But that's what you are probably worried about.
Assuming a UNIQUE or PK constraint on (col1,col2), you are dealing with a textbook UPSERT situation. Many related questions and answers to find here.
Generally, if any constraint is violated, an exception is raised which (unless trapped in subtransaction like it's possible in a procedural server-side language like plpgsql) will roll back not only the statement, but the whole transaction.
Without concurrent writes
I.e.: No other transactions will try to write to the same table at the same time.
Exclude rows that are already in the table with WHERE NOT EXISTS ... or any other applicable technique:
Select rows which are not present in other table
And don't forget to remove duplicates within the inserted set as well, which would not be excluded by the semi-anti-join WHERE NOT EXISTS ...
One technique to deal with both at once would be EXCEPT:
INSERT INTO tbl (col1, col2)
VALUES
(text 'v1', text 'v2') -- explicit type cast may be needed in 1st row
, ('v3', 'v4')
, ('v3', 'v4') -- beware of dupes in source
EXCEPT SELECT col1, col2 FROM tbl;
EXCEPT without the key word ALL folds duplicate rows in the source. If you know there are no dupes, or you don't want to fold duplicates silently, use EXCEPT ALL (or one of the other techniques). See:
Using EXCEPT clause in PostgreSQL
Generally, if the target table is big, WHERE NOT EXISTS in combination with DISTINCT on the source will probably be faster:
INSERT INTO tbl (col1, col2)
SELECT *
FROM (
SELECT DISTINCT *
FROM (
VALUES
(text 'v1', text'v2')
, ('v3', 'v4')
, ('v3', 'v4') -- dupes in source
) t(c1, c2)
) t
WHERE NOT EXISTS (
SELECT FROM tbl
WHERE col1 = t.c1 AND col2 = t.c2
);
If there can be many dupes, it pays to fold them in the source first. Else use one subquery less.
Related:
Select rows which are not present in other table
With concurrent writes
Use the Postgres UPSERT implementation INSERT ... ON CONFLICT ... in Postgres 9.5 or later:
INSERT INTO tbl (col1,col2)
SELECT DISTINCT * -- still can't insert the same row more than once
FROM (
VALUES
(text 'v1', text 'v2')
, ('v3','v4')
, ('v3','v4') -- you still need to fold dupes in source!
) t(c1, c2)
ON CONFLICT DO NOTHING; -- ignores rows with *any* conflict!
Further reading:
How to use RETURNING with ON CONFLICT in PostgreSQL?
How do I insert a row which contains a foreign key?
Documentation:
The manual
The commit page
The Postgres Wiki page
Craig's reference answer for UPSERT problems:
How to UPSERT (MERGE, INSERT ... ON DUPLICATE UPDATE) in PostgreSQL?
Will it stop the entire query and throw an exception? Yes.
To avoid that, you can look on the following SO question here, which describes how to avoid Postgres from throwing an error for multiple inserts when some of the inserted keys already exist on the DB.
You should basically do this:
INSERT INTO DBtable
(id, field1)
SELECT 1, 'value'
WHERE
NOT EXISTS (
SELECT id FROM DBtable WHERE id = 1
);
Sybase db tables do not have a concept of self updating row numbers. However , for one of the modules , I require the presence of rownumber corresponding to each row in the database such that max(Column) would always tell me the number of rows in the table.
I thought I'll introduce an int column and keep updating this column to keep track of the row number. However I'm having problems in updating this column in case of deletes. What sql should I use in delete trigger to update this column?
You can easily assign a unique number to each row by using an identity column. The identity can be a numeric or an integer (in ASE12+).
This will almost do what you require. There are certain circumstances in which you will get a gap in the identity sequence. (These are called "identity gaps", the best discussion on them is here). Also deletes will cause gaps in the sequence as you've identified.
Why do you need to use max(col) to get the number of rows in the table, when you could just use count(*)? If you're trying to get the last row from the table, then you can do
select * from table where column = (select max(column) from table).
Regarding the delete trigger to update a manually managed column, I think this would be a potential source of deadlocks, and many performance issues. Imagine you have 1 million rows in your table, and you delete row 1, that's 999999 rows you now have to update to subtract 1 from the id.
Delete trigger
CREATE TRIGGER tigger ON myTable FOR DELETE
AS
update myTable
set id = id - (select count(*) from deleted d where d.id < t.id)
from myTable t
To avoid locking problems
You could add an extra table (which joins to your primary table) like this:
CREATE TABLE rowCounter
(id int, -- foreign key to main table
rownum int)
... and use the rownum field from this table.
If you put the delete trigger on this table then you would hugely reduce the potential for locking problems.
Approximate solution?
Does the table need to keep its rownumbers up to date all the time?
If not, you could have a job which runs every minute or so, which checks for gaps in the rownum, and does an update.
Question: do the rownumbers have to reflect the order in which rows were inserted?
If not, you could do far fewer updates, but only updating the most recent rows, "moving" them into gaps.
Leave a comment if you would like me to post any SQL for these ideas.
I'm not sure why you would want to do this. You could experiment with using temporary tables and "select into" with an Identity column like below.
create table test
(
col1 int,
col2 varchar(3)
)
insert into test values (100, "abc")
insert into test values (111, "def")
insert into test values (222, "ghi")
insert into test values (300, "jkl")
insert into test values (400, "mno")
select rank = identity(10), col1 into #t1 from Test
select * from #t1
delete from test where col2="ghi"
select rank = identity(10), col1 into #t2 from Test
select * from #t2
drop table test
drop table #t1
drop table #t2
This would give you a dynamic id (of sorts)