PostgreSQL – Does It Preserve Insertion Order of Records?

insertorder-bypostgresqlpostgresql-9.4

For example when I'm using query which returns record ids

INSERT INTO projects(name)
VALUES (name1), (name2), (name3) returning id;

Which produce output:

1
2
3

Will this ids point to corresponding inserted values?

1 -> name1
2 -> name2
3 -> name3

Best Answer

The answer for this simple case is Yes. Rows are inserted in the provided order in the VALUES expression. And if your id column is a serial type, values from the underlying sequence will be fetched in that order.

But this is an implementation detail and there are no guarantees. In particular, the order is not necessarily maintained in more complex queries with WHERE conditions or joins.

You might also get gaps or other rows mixed in if you have concurrent transactions writing to the same table at the same time. Unlikely, but possible.

There is no "natural" order in a database table. While the physical order of rows (which is reflected in the system column ctid) will correspond to their inserted order initially, that may change any time. UPDATE, DELETE, VACUUM and other commands can change the physical order of rows. But the generated values for id are stable and not in any way connected to that, of course.

Related Solutions

Sql-server – Select one name per person where spelling variations exist

You can "group by" Emp_ID and use an aggregate function like MIN() or MAX() to get one of the names:

INSERT INTO TargetTable 
  (Emp_ID, Name) 
SELECT Emp_ID, MIN(Name) 
FROM SourceTable 
GROUP BY Emp_ID ;

And note that there is no inherent order in a table (actually you can define a clustered index for a table and this affects how the rows are stored on the disk but that is no guarantee for the order of retrieval).

You can get the data from the target table with a query afterwards and if you want them ordered, you can (and should) define the order you like (and a different one if you like, every time you query it):

SELECT Emp_ID, Name 
FROM TargetTable 
ORDER BY Name ;

Postgresql – Possible to have nested inserts in Postgres 8.4

You should be able to do something like this with a writable CTE:

WITH i AS (
   INSERT INTO host (hostname, hostrole) VALUES ('foobar', 'Virtual') RETURNING id
)
INSERT INTO interface (name, mac, host)
SELECT 'eth0', '00:50:56:9d:34:d4', id
FROM i

(untested, but it should be something like that)

Writable CTE is in PostgreSQL 9.1 and up.

Best Answer

Related Solutions

Sql-server – Select one name per person where spelling variations exist

Postgresql – Possible to have nested inserts in Postgres 8.4

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