I'm looking for a solution to a simple scenario. I need to check if a value is present in a table, and if present I need Y else N
I can do it in two ways, either fetch the count of rows from the database, and code the logic in java, or use DECODE(COUNT(*),0,'N','Y')
Which is better? Is there any advantage of one over the other? Or more specifically, is there any disadvantage of using DECODE() instead of doing it in Java?
The database I have is DB2.
You should use exists. I would tend to do this as:
select (case when exists (select 1 from . . . .)
then 'Y' else 'N'
end) as flag
from sysibm.sysdummy1;
The reason you want to use exists is because it is faster. When you use count(*), the SQL engine has to process all the (appropriate) data to get the count. With exists, it can stop at the first one.
The reason to prefer case over decode() is that the former is ANSI standard SQL, available in basically all databases.
It shouldn't be any considerable difference between those 2 ways that you mentioned.
1) The DECODE will be simple and the IF will be simple.
2) You will be receiving an Int32 versus a CHAR(1) - which is not a significant difference.
So, I would consider another aspect: Which of those 2 will make your code more CLEAR?
And one more thing: if this is the ONLY thing that you're selecting on that query, you could try something like:
SELECT 'Y' FROM DUAL WHERE EXISTS (SELECT 1 FROM YOURTABLE WHERE YOURCONDITION = 1); --Oracle SQL - but should be fairly easy to translate it to DB2
This is an option to not make the DB count for every occurrence of your condition just to check if it exists.
Aggregated functions like count can be optimized with MQT - Materilized Query Tables
https://www.ibm.com/developerworks/data/library/techarticle/dm-0509melnyk/
connect to sample
alter table employee add unique (empno)
alter table department add unique (deptno)
create table count_emp_dpto_1 as (select d.deptno, e.empno, count(*) from employee e, department d where d.deptno = 1 and e.workdept = d.deptno) data initially deferred refresh immediate
set integrity for count_emp_dpto_1 immediate checked not incremental
select * from count_emp_dpto_1
connect reset
I have an entity that has a CLOB attribute:
public class EntityS {
...
#Lob
private String description;
}
To retrieve certain EntityS from the DB we use a CriteriaQuery where we need the results to be unique, so we do:
query.where(builder.and(predicates.toArray(new Predicate[predicates.size()]))).distinct(true).orderBy(builder.asc(root.<Long> get(EntityS_.id)));
If we do that we get the following error:
ORA-00932: inconsistent datatypes: expected - got CLOB
I know that's because you cannot use distinct when selecting a CLOB. But we need the CLOB. Is there a workaround for this using CriteriaQuery with Predicates and so on?
We are using an ugly workaround getting rid of the .unique(true) and then filtering the results, but that's crap. We are using it only to be able to keep on developing the app, but we need a better solution and I don't seem to find one...
In case you are using Hibernate as persistence provider, you can specify the following query hint:
query.setHint(QueryHints.HINT_PASS_DISTINCT_THROUGH, false);
This way, "distinct" is not passed through to the SQL command, but Hibernate will take care of returning only distinct values.
See here for more information: https://thoughts-on-java.org/hibernate-tips-apply-distinct-to-jpql-but-not-sql-query/
Thinking outside the box - I have no idea if this will work, but perhaps it is worth a shot. (I tested it and it seems to work, but I created a table with just one column, CLOB data type, and two rows, both with the value to_clob('abcd') - of course it should work on that setup.)
To de-duplicate, compute a hash of each clob, and instruct Oracle to compute a row number partitioned by the hash value and ordered by nothing (null). Then select just the rows where the row number is 1. Something like below (t is the table I created, with one CLOB column called c).
I expect that execution time should be reasonably good. The biggest concern, of course, is collisions. How important is it that you not miss ANY of the CLOBs, and how many rows do you have in the base table in the first place? Is something like "one chance in a billion" of having a collision acceptable?
select c
from (
select c, row_number() over (partition by dbms_crypto.hash(c, 3) order by null) as rn
from t
)
where rn = 1;
Note - the user (your application, in your case) must have EXECUTE privilege on SYS.DBMS_CRYPTO. A DBA can grant it if needed.
Several months ago I learned from an answer on Stack Overflow how to perform multiple updates at once in MySQL using the following syntax:
INSERT INTO table (id, field, field2) VALUES (1, A, X), (2, B, Y), (3, C, Z)
ON DUPLICATE KEY UPDATE field=VALUES(Col1), field2=VALUES(Col2);
I've now switched over to PostgreSQL and apparently this is not correct. It's referring to all the correct tables so I assume it's a matter of different keywords being used but I'm not sure where in the PostgreSQL documentation this is covered.
To clarify, I want to insert several things and if they already exist to update them.
PostgreSQL since version 9.5 has UPSERT syntax, with ON CONFLICT clause. with the following syntax (similar to MySQL)
INSERT INTO the_table (id, column_1, column_2)
VALUES (1, 'A', 'X'), (2, 'B', 'Y'), (3, 'C', 'Z')
ON CONFLICT (id) DO UPDATE
SET column_1 = excluded.column_1,
column_2 = excluded.column_2;
Searching postgresql's email group archives for "upsert" leads to finding an example of doing what you possibly want to do, in the manual:
Example 38-2. Exceptions with UPDATE/INSERT
This example uses exception handling to perform either UPDATE or INSERT, as appropriate:
CREATE TABLE db (a INT PRIMARY KEY, b TEXT);
CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
LOOP
-- first try to update the key
-- note that "a" must be unique
UPDATE db SET b = data WHERE a = key;
IF found THEN
RETURN;
END IF;
-- not there, so try to insert the key
-- if someone else inserts the same key concurrently,
-- we could get a unique-key failure
BEGIN
INSERT INTO db(a,b) VALUES (key, data);
RETURN;
EXCEPTION WHEN unique_violation THEN
-- do nothing, and loop to try the UPDATE again
END;
END LOOP;
END;
$$
LANGUAGE plpgsql;
SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');
There's possibly an example of how to do this in bulk, using CTEs in 9.1 and above, in the hackers mailing list:
WITH foos AS (SELECT (UNNEST(%foo[])).*)
updated as (UPDATE foo SET foo.a = foos.a ... RETURNING foo.id)
INSERT INTO foo SELECT foos.* FROM foos LEFT JOIN updated USING(id)
WHERE updated.id IS NULL;
See a_horse_with_no_name's answer for a clearer example.
Warning: this is not safe if executed from multiple sessions at the same time (see caveats below).
Another clever way to do an "UPSERT" in postgresql is to do two sequential UPDATE/INSERT statements that are each designed to succeed or have no effect.
UPDATE table SET field='C', field2='Z' WHERE id=3;
INSERT INTO table (id, field, field2)
SELECT 3, 'C', 'Z'
WHERE NOT EXISTS (SELECT 1 FROM table WHERE id=3);
The UPDATE will succeed if a row with "id=3" already exists, otherwise it has no effect.
The INSERT will succeed only if row with "id=3" does not already exist.
You can combine these two into a single string and run them both with a single SQL statement execute from your application. Running them together in a single transaction is highly recommended.
This works very well when run in isolation or on a locked table, but is subject to race conditions that mean it might still fail with duplicate key error if a row is inserted concurrently, or might terminate with no row inserted when a row is deleted concurrently. A SERIALIZABLE transaction on PostgreSQL 9.1 or higher will handle it reliably at the cost of a very high serialization failure rate, meaning you'll have to retry a lot. See why is upsert so complicated, which discusses this case in more detail.
This approach is also subject to lost updates in read committed isolation unless the application checks the affected row counts and verifies that either the insert or the update affected a row.
With PostgreSQL 9.1 this can be achieved using a writeable CTE (common table expression):
WITH new_values (id, field1, field2) as (
values
(1, 'A', 'X'),
(2, 'B', 'Y'),
(3, 'C', 'Z')
),
upsert as
(
update mytable m
set field1 = nv.field1,
field2 = nv.field2
FROM new_values nv
WHERE m.id = nv.id
RETURNING m.*
)
INSERT INTO mytable (id, field1, field2)
SELECT id, field1, field2
FROM new_values
WHERE NOT EXISTS (SELECT 1
FROM upsert up
WHERE up.id = new_values.id)
See these blog entries:
Upserting via Writeable CTE
WAITING FOR 9.1 – WRITABLE CTE
WHY IS UPSERT SO COMPLICATED?
Note that this solution does not prevent a unique key violation but it is not vulnerable to lost updates.
See the follow up by Craig Ringer on dba.stackexchange.com
In PostgreSQL 9.5 and newer you can use INSERT ... ON CONFLICT UPDATE.
See the documentation.
A MySQL INSERT ... ON DUPLICATE KEY UPDATE can be directly rephrased to a ON CONFLICT UPDATE. Neither is SQL-standard syntax, they're both database-specific extensions. There are good reasons MERGE wasn't used for this, a new syntax wasn't created just for fun. (MySQL's syntax also has issues that mean it wasn't adopted directly).
e.g. given setup:
CREATE TABLE tablename (a integer primary key, b integer, c integer);
INSERT INTO tablename (a, b, c) values (1, 2, 3);
the MySQL query:
INSERT INTO tablename (a,b,c) VALUES (1,2,3)
ON DUPLICATE KEY UPDATE c=c+1;
becomes:
INSERT INTO tablename (a, b, c) values (1, 2, 10)
ON CONFLICT (a) DO UPDATE SET c = tablename.c + 1;
Differences:
You must specify the column name (or unique constraint name) to use for the uniqueness check. That's the ON CONFLICT (columnname) DO
The keyword SET must be used, as if this was a normal UPDATE statement
It has some nice features too:
You can have a WHERE clause on your UPDATE (letting you effectively turn ON CONFLICT UPDATE into ON CONFLICT IGNORE for certain values)
The proposed-for-insertion values are available as the row-variable EXCLUDED, which has the same structure as the target table. You can get the original values in the table by using the table name. So in this case EXCLUDED.c will be 10 (because that's what we tried to insert) and "table".c will be 3 because that's the current value in the table. You can use either or both in the SET expressions and WHERE clause.
For background on upsert see How to UPSERT (MERGE, INSERT ... ON DUPLICATE UPDATE) in PostgreSQL?
I was looking for the same thing when I came here, but the lack of a generic "upsert" function botherd me a bit so I thought you could just pass the update and insert sql as arguments on that function form the manual
that would look like this:
CREATE FUNCTION upsert (sql_update TEXT, sql_insert TEXT)
RETURNS VOID
LANGUAGE plpgsql
AS $$
BEGIN
LOOP
-- first try to update
EXECUTE sql_update;
-- check if the row is found
IF FOUND THEN
RETURN;
END IF;
-- not found so insert the row
BEGIN
EXECUTE sql_insert;
RETURN;
EXCEPTION WHEN unique_violation THEN
-- do nothing and loop
END;
END LOOP;
END;
$$;
and perhaps to do what you initially wanted to do, batch "upsert", you could use Tcl to split the sql_update and loop the individual updates, the preformance hit will be very small see http://archives.postgresql.org/pgsql-performance/2006-04/msg00557.php
the highest cost is executing the query from your code, on the database side the execution cost is much smaller
There is no simple command to do it.
The most correct approach is to use function, like the one from docs.
Another solution (although not that safe) is to do update with returning, check which rows were updates, and insert the rest of them
Something along the lines of:
update table
set column = x.column
from (values (1,'aa'),(2,'bb'),(3,'cc')) as x (id, column)
where table.id = x.id
returning id;
assuming id:2 was returned:
insert into table (id, column) values (1, 'aa'), (3, 'cc');
Of course it will bail out sooner or later (in concurrent environment), as there is clear race condition in here, but usually it will work.
Here's a longer and more comprehensive article on the topic.
I use this function merge
CREATE OR REPLACE FUNCTION merge_tabla(key INT, data TEXT)
RETURNS void AS
$BODY$
BEGIN
IF EXISTS(SELECT a FROM tabla WHERE a = key)
THEN
UPDATE tabla SET b = data WHERE a = key;
RETURN;
ELSE
INSERT INTO tabla(a,b) VALUES (key, data);
RETURN;
END IF;
END;
$BODY$
LANGUAGE plpgsql
Personally, I've set up a "rule" attached to the insert statement. Say you had a "dns" table that recorded dns hits per customer on a per-time basis:
CREATE TABLE dns (
"time" timestamp without time zone NOT NULL,
customer_id integer NOT NULL,
hits integer
);
You wanted to be able to re-insert rows with updated values, or create them if they didn't exist already. Keyed on the customer_id and the time. Something like this:
CREATE RULE replace_dns AS
ON INSERT TO dns
WHERE (EXISTS (SELECT 1 FROM dns WHERE ((dns."time" = new."time")
AND (dns.customer_id = new.customer_id))))
DO INSTEAD UPDATE dns
SET hits = new.hits
WHERE ((dns."time" = new."time") AND (dns.customer_id = new.customer_id));
Update: This has the potential to fail if simultaneous inserts are happening, as it will generate unique_violation exceptions. However, the non-terminated transaction will continue and succeed, and you just need to repeat the terminated transaction.
However, if there are tons of inserts happening all the time, you will want to put a table lock around the insert statements: SHARE ROW EXCLUSIVE locking will prevent any operations that could insert, delete or update rows in your target table. However, updates that do not update the unique key are safe, so if you no operation will do this, use advisory locks instead.
Also, the COPY command does not use RULES, so if you're inserting with COPY, you'll need to use triggers instead.
Similar to most-liked answer, but works slightly faster:
WITH upsert AS (UPDATE spider_count SET tally=1 WHERE date='today' RETURNING *)
INSERT INTO spider_count (spider, tally) SELECT 'Googlebot', 1 WHERE NOT EXISTS (SELECT * FROM upsert)
(source: http://www.the-art-of-web.com/sql/upsert/)
I custom "upsert" function above, if you want to INSERT AND REPLACE :
`
CREATE OR REPLACE FUNCTION upsert(sql_insert text, sql_update text)
RETURNS void AS
$BODY$
BEGIN
-- first try to insert and after to update. Note : insert has pk and update not...
EXECUTE sql_insert;
RETURN;
EXCEPTION WHEN unique_violation THEN
EXECUTE sql_update;
IF FOUND THEN
RETURN;
END IF;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
ALTER FUNCTION upsert(text, text)
OWNER TO postgres;`
And after to execute, do something like this :
SELECT upsert($$INSERT INTO ...$$,$$UPDATE... $$)
Is important to put double dollar-comma to avoid compiler errors
check the speed...
According the PostgreSQL documentation of the INSERT statement, handling the ON DUPLICATE KEY case is not supported. That part of the syntax is a proprietary MySQL extension.
I have the same issue for managing account settings as name value pairs.
The design criteria is that different clients could have different settings sets.
My solution, similar to JWP is to bulk erase and replace, generating the merge record within your application.
This is pretty bulletproof, platform independent and since there are never more than about 20 settings per client, this is only 3 fairly low load db calls - probably the fastest method.
The alternative of updating individual rows - checking for exceptions then inserting - or some combination of is hideous code, slow and often breaks because (as mentioned above) non standard SQL exception handling changing from db to db - or even release to release.
#This is pseudo-code - within the application:
BEGIN TRANSACTION - get transaction lock
SELECT all current name value pairs where id = $id into a hash record
create a merge record from the current and update record
(set intersection where shared keys in new win, and empty values in new are deleted).
DELETE all name value pairs where id = $id
COPY/INSERT merged records
END TRANSACTION
CREATE OR REPLACE FUNCTION save_user(_id integer, _name character varying)
RETURNS boolean AS
$BODY$
BEGIN
UPDATE users SET name = _name WHERE id = _id;
IF FOUND THEN
RETURN true;
END IF;
BEGIN
INSERT INTO users (id, name) VALUES (_id, _name);
EXCEPTION WHEN OTHERS THEN
UPDATE users SET name = _name WHERE id = _id;
END;
RETURN TRUE;
END;
$BODY$
LANGUAGE plpgsql VOLATILE STRICT
For merging small sets, using the above function is fine. However, if you are merging large amounts of data, I'd suggest looking into http://mbk.projects.postgresql.org
The current best practice that I'm aware of is:
COPY new/updated data into temp table (sure, or you can do INSERT if the cost is ok)
Acquire Lock [optional] (advisory is preferable to table locks, IMO)
Merge. (the fun part)
UPDATE will return the number of modified rows. If you use JDBC (Java), you can then check this value against 0 and, if no rows have been affected, fire INSERT instead. If you use some other programming language, maybe the number of the modified rows still can be obtained, check documentation.
This may not be as elegant but you have much simpler SQL that is more trivial to use from the calling code. Differently, if you write the ten line script in PL/PSQL, you probably should have a unit test of one or another kind just for it alone.
Edit: This does not work as expected. Unlike the accepted answer, this produces unique key violations when two processes repeatedly call upsert_foo concurrently.
Eureka! I figured out a way to do it in one query: use UPDATE ... RETURNING to test if any rows were affected:
CREATE TABLE foo (k INT PRIMARY KEY, v TEXT);
CREATE FUNCTION update_foo(k INT, v TEXT)
RETURNS SETOF INT AS $$
UPDATE foo SET v = $2 WHERE k = $1 RETURNING $1
$$ LANGUAGE sql;
CREATE FUNCTION upsert_foo(k INT, v TEXT)
RETURNS VOID AS $$
INSERT INTO foo
SELECT $1, $2
WHERE NOT EXISTS (SELECT update_foo($1, $2))
$$ LANGUAGE sql;
The UPDATE has to be done in a separate procedure because, unfortunately, this is a syntax error:
... WHERE NOT EXISTS (UPDATE ...)
Now it works as desired:
SELECT upsert_foo(1, 'hi');
SELECT upsert_foo(1, 'bye');
SELECT upsert_foo(3, 'hi');
SELECT upsert_foo(3, 'bye');
PostgreSQL >= v15
Big news on this topic as in PostgreSQL v15, it is possible to use MERGE command. In fact, this long awaited feature was listed the first of the improvements of the v15 release.
This is similar to INSERT ... ON CONFLICT but more batch-oriented. It has a powerful WHEN MATCHED vs WHEN NOT MATCHED structure that gives the ability to INSERT, UPDATE or DELETE on such conditions.
It not only eases bulk changes, but it even adds more control that tradition UPSERT and INSERT ... ON CONFLICT
Take a look at this very complete sample from official page:
MERGE INTO wines w
USING wine_stock_changes s
ON s.winename = w.winename
WHEN NOT MATCHED AND s.stock_delta > 0 THEN
INSERT VALUES(s.winename, s.stock_delta)
WHEN MATCHED AND w.stock + s.stock_delta > 0 THEN
UPDATE SET stock = w.stock + s.stock_delta
WHEN MATCHED THEN
DELETE;
PostgreSQL v9, v10, v11, v12, v13, v14
If version is under v15 and over v9.5 , probably best choice is to use UPSERT syntax, with ON CONFLICT clause
Here is the example how to do upsert with params and without special sql constructions
if you have special condition (sometimes you can't use 'on conflict' because you can't create constraint)
WITH upd AS
(
update view_layer set metadata=:metadata where layer_id = :layer_id and view_id = :view_id returning id
)
insert into view_layer (layer_id, view_id, metadata)
(select :layer_id layer_id, :view_id view_id, :metadata metadata FROM view_layer l
where NOT EXISTS(select id FROM upd WHERE id IS NOT NULL) limit 1)
returning id
maybe it will be helpful
I'm working with JDBC and HSQLDB 2.2.9. What's the most efficient and accurate way to insert a new row into a DB and, subsequently, retain its id (PK set to autoincrement) value? The reason I need to do this is probably pretty obvious, but I'll illustrate with an example for discussion:
Say there's a Customer table that has a PersonId field with a FK constraint referring to a row from a Person table. I want to create a new Customer, but to do this I need to first create a new Person and use the new Person.id value to set Customer.PersonId.
I've seen four ways to approach this:
Insert the Person row setting the id field to null. HSQLDB generates the next id value automatically. Then perform a query on the Person table to get the id value just created and use it to create the new Customer row.
This seems expensive just to retrieve a single integer value.
Get the next id value in the Person table and use it in the INSERT statement to set the Person.id value manually. Use the same id value to set Customer.PersonId. No subsequent read from the DB is needed.
Inconsistencies may arise if an id value is obtained, but another connection performs an INSERT in the table before my INSERT INTO Person... statement is executed.
Execute the INSERT statement, as in option 1 above, setting id=null to allow auto-generation. Then use the getGeneratedKeys method to retrieve keys generated in last statement.
I thought this sounded like a good option, but I couldn't get it to work. Here's a snippet of my code:
// PreparedStatement prepared previously...
preparedStatement.executeUpdate();
ResultSet genKeys = preparedStatement.getGeneratedKeys();
int id;
if (genKeys.next()) {
id = genKeys.getInt(1);
}
// Finish up method...
This code was returning an empty ResultSet for genKeys. Am I using the getGeneratedKeys method incorrectly? If I could get this to work, this might be the way to go.
Again, execute the INSERT statement allowing for auto-generated id. Then immediately execute CALL IDENTITY() to retrieve the last id value generated by the connection (as explained here and mentioned in this SO question).
This also seems like a reasonable option, even though I must perform an additional executeQuery. On the positive side, I was actually able to get it to work with the following code:
// INSERT statement executed here...
statement = connection.createStatement();
ResultSet rs = statement.executeQuery("CALL IDENTITY();");
int id;
if (rs.next()) id = rs.getInt(1);
// Finish up method...
So, in summary, the first two options I'm not crazy about. The second two seem ok, but I could only get option 4 to work. Which option is preferred and why? If option 3 is the best, what am I doing wrong? Also, is there a better way that I haven't mentioned? I know words like 'better' can be subjective, but I'm working with a simple DB and want the most direct solution that doesn't open up the DB to possible inconsistencies and doesn't increase the transaction failure rate (due to trying to create a record with an id that already exists).
This seems like a basic question (and essential), but I couldn't find much guidance on the best way to do it. Thanks.
EDIT:
I just found this question that discusses my option 3. According to the accepted answer, it appears I was leaving out the Statement.RETURN_GENERATED_KEYS parameter needed to enable that functionality. I didn't show the prepareStatement method in my code snippet, but I was using the single parameter version. I need to retry using the overloaded, two-parameter version.
There are also a few other SO questions which show up with that question that are closly related to my question. So, I guess mine could be considered a duplicate (not sure how I missed those other questions before). But I'd still like any guidance on whether one solution is considered better than the others. For now, if I get option 3 to work, I'll probably go with that.
I don't have enough reputation to comment on neizan's answer, but here's how I solved the same problem:
The column looked like an ID column, but it wasn't defined as IDENTITY;
As said above, you need to specify RETURN_GENERATED_KEYS.
It looks like if you execute 2 INSERT in sequence, the second one won't return the generated keys. Use "CALL IDENTITY()" instead.
Example using HSQLDB 2.2.9:
CREATE TABLE MY_TABLE (
ID INTEGER IDENTITY,
NAME VARCHAR(30)
)
Then in Java:
PreparedStatement result = cnx.prepareStatement(
"INSERT INTO MY_TABLE(ID, NAME) VALUES(NULL, 'TOM');",
RETURN_GENERATED_KEYS);
int updated = result.executeUpdate();
if (updated == 1) {
ResultSet generatedKeys = result.getGeneratedKeys();
if (generatedKeys.next()) {
int key = generatedKeys.getInt(1);
}
}
Not much action here, so I'll go ahead and answer to bring closure to this question. After playing around with the different options, and after see this question, I was able to get my option 3 to work. Like I mentioned in the edit to my question, I'm going to use option 3. Option 4 also worked fine, but since the accepted answer to the linked question is given by a reputable source, I am sticking with that. I wish I'd have seen that question/answer before starting this one, I'd have saved some time!