Is Select or Insert in a Function Prone to Race Conditions

Is SELECT or INSERT in a function prone to race conditions?

It's the recurring problem of SELECT or INSERT under possible concurrent write load, related to (but different from) UPSERT (which is INSERT or UPDATE).

This PL/pgSQL function uses UPSERT (INSERT ... ON CONFLICT .. DO UPDATE) to INSERT or SELECT a single row:

CREATE OR REPLACE FUNCTION f_tag_id(_tag text, OUT _tag_id int)
  LANGUAGE plpgsql AS
$func$
BEGIN
   SELECT tag_id  -- only if row existed before
   FROM   tag
   WHERE  tag = _tag
   INTO   _tag_id;

   IF NOT FOUND THEN
      INSERT INTO tag AS t (tag)
      VALUES (_tag)
      ON     CONFLICT (tag) DO NOTHING
      RETURNING t.tag_id
      INTO   _tag_id;
   END IF;
END
$func$;

There is still a tiny window for a race condition. To make absolutely sure we get an ID:

CREATE OR REPLACE FUNCTION f_tag_id(_tag text, OUT _tag_id int)
  LANGUAGE plpgsql AS
$func$
BEGIN
   LOOP
      SELECT tag_id
      FROM   tag
      WHERE  tag = _tag
      INTO   _tag_id;

      EXIT WHEN FOUND;

      INSERT INTO tag AS t (tag)
      VALUES (_tag)
      ON     CONFLICT (tag) DO NOTHING
      RETURNING t.tag_id
      INTO   _tag_id;

      EXIT WHEN FOUND;
   END LOOP;
END
$func$;

db<>fiddle here

This keeps looping until either INSERT or SELECT succeeds.
Call:

SELECT f_tag_id('possibly_new_tag');

If subsequent commands in the same transaction rely on the existence of the row and it is actually possible that other transactions update or delete it concurrently, you can lock an existing row in the SELECT statement with FOR SHARE.

If the row gets inserted instead, it is locked (or not visible for other transactions) until the end of the transaction anyway.

Start with the common case (INSERT vs SELECT) to make it faster.

Related:

• Get Id from a conditional INSERT
• How to include excluded rows in RETURNING from INSERT ... ON CONFLICT

Related (pure SQL) solution to INSERT or SELECT multiple rows (a set) at once:

• How to use RETURNING with ON CONFLICT in PostgreSQL?

What's wrong with this pure SQL solution?

CREATE OR REPLACE FUNCTION f_tag_id(_tag text, OUT _tag_id int)
  LANGUAGE sql AS
$func$
WITH ins AS (
   INSERT INTO tag AS t (tag)
   VALUES (_tag)
   ON     CONFLICT (tag) DO NOTHING
   RETURNING t.tag_id
   )
SELECT tag_id FROM ins
UNION  ALL
SELECT tag_id FROM tag WHERE tag = _tag
LIMIT  1;
$func$;

Not entirely wrong, but it fails to seal a loophole, like @FunctorSalad worked out. The function can come up with an empty result if a concurrent transaction tries to do the same at the same time. The manual:

All the statements are executed with the same snapshot

If a concurrent transaction inserts the same new tag a moment earlier, but hasn't committed, yet:

• The UPSERT part comes up empty, after waiting for the concurrent transaction to finish. (If the concurrent transaction should roll back, it still inserts the new tag and returns a new ID.)

• The SELECT part also comes up empty, because it's based on the same snapshot, where the new tag from the (yet uncommitted) concurrent transaction is not visible.

We get nothing. Not as intended. That's counter-intuitive to naive logic (and I got caught there), but that's how the MVCC model of Postgres works - has to work.

So do not use this if multiple transactions can try to insert the same tag at the same time. Or loop until you actually get a row. The loop will hardly ever be triggered in common work loads anyway.

Postgres 9.4 or older

Given this (slightly simplified) table:

CREATE table tag (
  tag_id serial PRIMARY KEY
, tag    text   UNIQUE
);

An almost 100% secure function to insert new tag / select existing one, could look like this.

CREATE OR REPLACE FUNCTION f_tag_id(_tag text, OUT tag_id int)
  LANGUAGE plpgsql AS
$func$
BEGIN
   LOOP
      BEGIN
      WITH sel AS (SELECT t.tag_id FROM tag t WHERE t.tag = _tag FOR SHARE)
         , ins AS (INSERT INTO tag(tag)
                   SELECT _tag
                   WHERE  NOT EXISTS (SELECT 1 FROM sel)  -- only if not found
                   RETURNING tag.tag_id)       -- qualified so no conflict with param
      SELECT sel.tag_id FROM sel
      UNION  ALL
      SELECT ins.tag_id FROM ins
      INTO   tag_id;

      EXCEPTION WHEN UNIQUE_VIOLATION THEN     -- insert in concurrent session?
         RAISE NOTICE 'It actually happened!'; -- hardly ever happens
      END;

      EXIT WHEN tag_id IS NOT NULL;            -- else keep looping
   END LOOP;
END
$func$;

db<>fiddle here

_{Old sqlfiddle}

Why not 100%? Consider the notes in the manual for the related UPSERT example:

• https://www.postgresql.org/docs/current/plpgsql-control-structures.html#PLPGSQL-UPSERT-EXAMPLE

Explanation

• Try the SELECT first. This way you avoid the considerably more expensive exception handling 99.99% of the time.

• Use a CTE to minimize the (already tiny) time slot for the race condition.

• The time window between the SELECT and the INSERT within one query is super tiny. If you don't have heavy concurrent load, or if you can live with an exception once a year, you could just ignore the case and use the SQL statement, which is faster.

• No need for FETCH FIRST ROW ONLY (= LIMIT 1). The tag name is obviously UNIQUE.

• Remove FOR SHARE in my example if you don't usually have concurrent DELETE or UPDATE on the table tag. Costs a tiny bit of performance.

• Never quote the language name: 'plpgsql'. plpgsql is an identifier. Quoting may cause problems and is only tolerated for backwards compatibility.

• Don't use non-descriptive column names like id or name. When joining a couple of tables (which is what you do in a relational DB) you end up with multiple identical names and have to use aliases.

Built into your function

Using this function you could largely simplify your FOREACH LOOP to:

...
FOREACH TagName IN ARRAY $3
LOOP
   INSERT INTO taggings (PostId, TagId)
   VALUES   (InsertedPostId, f_tag_id(TagName));
END LOOP;
...

Faster, though, as a single SQL statement with unnest():

INSERT INTO taggings (PostId, TagId)
SELECT InsertedPostId, f_tag_id(tag)
FROM   unnest($3) tag;

Replaces the whole loop.

Alternative solution

This variant builds on the behavior of UNION ALL with a LIMIT clause: as soon as enough rows are found, the rest is never executed:

• Way to try multiple SELECTs till a result is available?

Building on this, we can outsource the INSERT into a separate function. Only there we need exception handling. Just as safe as the first solution.

CREATE OR REPLACE FUNCTION f_insert_tag(_tag text, OUT tag_id int)
  RETURNS int
  LANGUAGE plpgsql AS
$func$
BEGIN
   INSERT INTO tag(tag) VALUES (_tag) RETURNING tag.tag_id INTO tag_id;

   EXCEPTION WHEN UNIQUE_VIOLATION THEN  -- catch exception, NULL is returned
END
$func$;

Which is used in the main function:

CREATE OR REPLACE FUNCTION f_tag_id(_tag text, OUT _tag_id int)
   LANGUAGE plpgsql AS
$func$
BEGIN
   LOOP
      SELECT tag_id FROM tag WHERE tag = _tag
      UNION  ALL
      SELECT f_insert_tag(_tag)  -- only executed if tag not found
      LIMIT  1  -- not strictly necessary, just to be clear
      INTO   _tag_id;

      EXIT WHEN _tag_id IS NOT NULL;  -- else keep looping
   END LOOP;
END
$func$;

• This is a bit cheaper if most of the calls only need SELECT, because the more expensive block with INSERT containing the EXCEPTION clause is rarely entered. The query is also simpler.

• FOR SHARE is not possible here (not allowed in UNION query).

• LIMIT 1 would not be necessary (tested in pg 9.4). Postgres derives LIMIT 1 from INTO _tag_id and only executes until the first row is found.

Can an INSERT-SELECT query be subject to race conditions?

Yes it will fail with a duplicate key value violates unique constraint error. What I do is to place the insertion code in a try/except block and when the exception is thrown I catch it and retry. That simple. Unless the application has a huge amount of users it will work flawlessly.

In your query the default isolation level is enough since it is a single insert statement and there is no risk of phantom reads.

Notice that even when setting the isolation level to serializable the try/except block is not avoidable. From the manual about serializable:

like the Repeatable Read level, applications using this level must be prepared to retry transactions due to serialization failures

Is (insert into ... select) statement safe about race conditions - SQL Server?

Look at the locking on the table(s), and hints used. Based on the information provided I expect this to be open to race conditions (i.e. a lock is not maintained on the table). You can look at setting-up locks with SQL hits, or transactions. There are a few posts out there that discuss locking. An example of one that looks pertinent to your question is:
SQL Server Table Lock during bulk insert

Without a continuous lock on the table(s) affected from the SELECT through the INSERT, yes you are open to a race condition.

NOTE - locking the table for extended periods will impact performance to varying degrees depending on the volume of traffic, and time locked (if a lot of users are trying to book at the same time, 1sec difference can become significant). Therefore, if you do lock the table, I recommend spending some time tuning your queries for performance. I wont say much on this as I do not have any metrics on your data, or know your indexes. Probably best to start by simply re-structuring your queries to hit the most restrictive table first. I assume this would be the table holding alloted_room_id (pass the ID so it can be used in the WHERE clause).

Race Condition between SELECT and INSERT for multiple columns

The wrench in the works is SELECT f_insert_tag(tag_p_id, _tag) instead of

SELECT * FROM f_insert_tag(tag_p_id, _tag)

For Postgres 9.4

CREATE FUNCTION f_insert_tag(_tag_id int, _tag text, OUT _tag_id_ int, OUT _tag_ text) 
AS
$func$
 BEGIN
   INSERT INTO t(tag_id, tag)
   VALUES (_tag_id, _tag)
   RETURNING t.tag_id, t.tag
   INTO  _tag_id_, _tag_;

   EXCEPTION WHEN UNIQUE_VIOLATION THEN
      -- catch exception, return NULL
 END
$func$  LANGUAGE plpgsql;


CREATE FUNCTION f_tag_id(_tag_id int, _tag text, OUT _tag_id_ int, OUT _tag_ text) AS
$func$
BEGIN
LOOP
   SELECT t.tag_id, t.tag
   FROM   t
   WHERE  t.tag = _tag

   UNION ALL
   SELECT *                                              -- !!!
   FROM   f_insert_tag(_tag_id, _tag)
   LIMIT  1

   INTO _tag_id_, _tag_;

   EXIT WHEN _tag_id_ IS NOT NULL;  -- else keep looping
END LOOP;
END
$func$ LANGUAGE plpgsql;

db<>fiddle here

For Postgres 9.5 or later:

CREATE FUNCTION f_tag_id(_tag_id int, _tag text, OUT _tag_id_ int, OUT _tag_ text) AS
$func$
BEGIN
LOOP
   SELECT t.tag_id, t.tag
   FROM   t
   WHERE  t.tag = _tag
   INTO   _tag_id_, _tag_;

   EXIT WHEN FOUND;

   INSERT INTO t (tag_id, tag)
   VALUES (_tag_id, _tag)
   ON     CONFLICT (tag) DO NOTHING
   RETURNING t.tag_id, t.tag
   INTO   _tag_id_, _tag_;

   EXIT WHEN FOUND;
END LOOP;
END
$func$  LANGUAGE plpgsql;

db<>fiddle here

Basics here:

• Is SELECT or INSERT in a function prone to race conditions?

Race conditions between INSERT ON CONFLICT DO NOTHING and SELECT

To make absolutely sure that the single row in the first table is there, and it's ID returned, you could create a function like outlined here:

• Is SELECT or INSERT in a function prone to race conditions?

To make sure the row also stays there for the duration of the transaction, just make sure it's locked. If you INSERT the row, it's locked anyway. If you SELECT an existing id, you have to lock it explicitly - just like you suggested. FOR KEY SHARE is strong enough for our purpose as long as there is a (non-partial, non-functional) UNIQUE index on (scope, name), which is safe to assume given your ON CONFLICT clause.

CREATE OR REPLACE FUNCTION f_object_id(_scope text, _name text, OUT _object_id int)
  LANGUAGE plpgsql AS
$func$
BEGIN
LOOP
   SELECT id FROM object
   WHERE  scope = $1
   AND    name  = $2
   -- lock to prevent deletion in the tiny time frame before the next INSERT
   FOR    KEY SHARE
   INTO   _object_id;

   EXIT WHEN FOUND;

   INSERT INTO object AS o (scope, name)
   VALUES ($1, $2)
   ON     CONFLICT (scope, name) DO NOTHING
   RETURNING o.id
   INTO   _object_id;

   EXIT WHEN FOUND;
END LOOP;
END
$func$;

You really only need to lock the row if it's conceivable that a concurrent transaction might DELETE it (you don't UPDATE) in the tiny time frame between the SELECT and the next INSERT statement.

Also, if you have a FOREIGN KEY constraint from object_member.object_id to object.id (which seems likely), referential integrity is guaranteed anyway. If you don't add the explicit lock, and the row is deleted in between, you get a foreign key violation, and the INSERT to object_member is cancelled, along with the whole transaction. Else, the other transaction with the DELETE has to wait until your transaction is done, and is then cancelled by the same FK constraint since depending rows are now there (unless it's defined to CASCADE ...) So by locking (or not) you can decide whether to prevent the DELETE or the INSERT in this scenario.

Then your call burns down to just:

query(
  `WITH o(id) AS (SELECT f_object_id($1, $2))
   INSERT INTO object_member (object_id, key, value)
   SELECT o.id, UNNEST($3::text[]), UNNEST($4::int[])
   FROM   o;`
  [$scope, $name, $keys, $values]
)

Since you obviously insert multiple rows into object_member, I moved f_object_id($1, $2) to a CTE to avoid repeated execution - which would work, but pointlessly expensive.

In Postgres 12 or later I would make that explicit by adding MATERIALIZED (since the INSERT is hidden in a function):

WITH o(id) AS MATERIALIZED (SELECT f_object_id($1, $2)) ...

Aside: For the multiple unnest() in the SELECT list, make sure you are on Postgres 10 or later. See:

• What is the expected behaviour for multiple set-returning functions in SELECT clause?

Matters of detail

Will it make any difference (apart from execution time) to do this in the application logic with multiple queries in the same transaction?

Basically no. The only difference is performance. Well, and short code and reliability. It's objectively more error prone to go back and forth between db and client for each loop. But unless you have extremely competitive transactions, you would hardly ever be looping anyway.

The other consideration is this: the matter is tricky, and most developers do not understand it. Encapsulated in a server-side function, it's less likely to be broken by the next application programmer (or yourself). You have to make sure that it's actually used, too. Either way, properly document the reasons you are doing it one way or another ...

I really wonder whether my second snippet is safe, or why not (given the quote about visibility in the SELECT after the INSERT).

Mostly safe, but not absolutely. While the next separate SELECT will see (now committed) rows of a transactions competing with the previous UPSERT, there is nothing to keep a third transaction from deleting it again in the meantime. The row has not been locked, and you have no way to do that while it's not visible, and there is no generic predicate locking available in Postgres.

Consider this (T1, T2, T3 are concurrent transactions):

                               T2: BEGIN transaction
T1: BEGIN transaction
                               T2: INSERT object 666
T1: UPSERT object 666
    unique violation?
    -> wait for T2
                               T2: COMMIT
T1: unique violation -> NO ACTION
    finish statement
    can't return invisible object 666
                                             T3: DELETE object 666 & COMMIT
T1: SELECT object 666 -> no row!
    BOOM!

Typically it's extremely unlikely that it ever happens.

But it's possible. Hence the loop.

The other option is SERIALIZABLE transaction isolation. Typically more expensive, and you need to prepare for serialization failures. Catch 22.

How can I deal with a race condition in PostgreSQL?

Do it all in a single command:

result= """
INSERT INTO shortened_link ( url_shortened ...
SELECT %(url)s
where not exists (
    select 1
    from shortened_link
    WHERE url_shortened = %(url)s
);"""

It will only insert if that link does not exist.

Insert row if not exists leads to race condition?

There is an unavoidable "race" here, since two sessions cannot "see" eachothers uncommited rows. On a conflict, a session could only rollback (maybe to a savepoint) and retry. That would typically mean: referring to the other's freshly inserted row, instead of creating a private duplicate.

There is a data-modelling problem here: feed_channel appears to have a lot of candidate keys, and the cascading rule from feed_content could orphanise a lot of feed_content's rows (I suppose content-> channel is 1::M relation; more than one contents-row could refer to the same channel)

Finally, the feed_channel table at least needs the natural key {link,title}. That is where the insert/not exists is all about. (and the whole purpose of this function)

I cleaned up the function a bit. The IF construct is not needed, doing an INSERT WHERE NOT EXISTS first works just as well, and maybe even better.

DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp ;
SET search_path=tmp;

CREATE TABLE feed_channel
    ( id SERIAL PRIMARY KEY
    , name TEXT
    , link TEXT NOT NULL
    , title TEXT NOT NULL -- part of PK :: must be not nullable
    , CONSTRAINT feed_channel_nat UNIQUE (link,title) -- the natural key
);

CREATE TABLE feed_content
    ( id SERIAL PRIMARY KEY
    , channel INTEGER REFERENCES feed_channel(id) ON DELETE CASCADE ON UPDATE CASCADE
    , guid TEXT UNIQUE NOT NULL -- yet another primary key
    , title TEXT --
    , link TEXT  -- title && link appear to be yet another candidate key
    , description TEXT
    , pubdate TIMESTAMP
    );

-- NOTE: omitted original function channel_insert() for brevity
CREATE OR REPLACE FUNCTION channel_insert_wp(
  p_link feed_channel.link%TYPE,
  p_title feed_channel.title%TYPE
) RETURNS feed_channel.id%TYPE AS $body$
   DECLARE
    v_id feed_channel.id%TYPE;
  BEGIN
      INSERT INTO feed_channel(link,title)
      SELECT p_link,p_title
      WHERE NOT EXISTS ( SELECT *
        FROM feed_channel nx
        WHERE nx.link= p_link
        AND nx.title= p_title
        )
        ;
    SELECT id INTO v_id
    FROM feed_channel ex
    WHERE ex.link= p_link
    AND ex.title= p_title
        ;

    RETURN v_id;

  END;
$body$ LANGUAGE plpgsql;

SELECT channel_insert('Bogus_link', 'Bogus_title');
SELECT channel_insert_wp('Bogus_link2', 'Bogus_title2');

SELECT * FROM feed_channel;

Results:

DROP SCHEMA
CREATE SCHEMA
SET
NOTICE:  CREATE TABLE will create implicit sequence "feed_channel_id_seq" for serial column "feed_channel.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "feed_channel_pkey" for table "feed_channel"
NOTICE:  CREATE TABLE / UNIQUE will create implicit index "feed_channel_nat" for table "feed_channel"
CREATE TABLE
NOTICE:  CREATE TABLE will create implicit sequence "feed_content_id_seq" for serial column "feed_content.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "feed_content_pkey" for table "feed_content"
NOTICE:  CREATE TABLE / UNIQUE will create implicit index "feed_content_guid_key" for table "feed_content"
CREATE TABLE
NOTICE:  type reference feed_channel.link%TYPE converted to text
NOTICE:  type reference feed_channel.title%TYPE converted to text
NOTICE:  type reference feed_channel.id%TYPE converted to integer
CREATE FUNCTION
NOTICE:  type reference feed_channel.link%TYPE converted to text
NOTICE:  type reference feed_channel.title%TYPE converted to text
NOTICE:  type reference feed_channel.id%TYPE converted to integer
CREATE FUNCTION
 channel_insert 
----------------
              1
(1 row)

 channel_insert_wp 
-------------------
                 2
(1 row)

 id | name |    link     |    title     
----+------+-------------+--------------
  1 |      | Bogus_link  | Bogus_title
  2 |      | Bogus_link2 | Bogus_title2
(2 rows)

Race condition when insert a row

Yes, your situation may likely provoque a race condition.

Do you need these counters at all? You can easily replace them with appropriate queries:

SELECT COUNT(*) FROM ItemCatalog WHERE ID=100;
SELECT COUNT(*) FROM Item WHERE ID=100;

For successive field contents it is advisable to use AUTO_INCREMENT columns. But it seems that doesn't apply in your case.

But nevertheless, you can use the COUNT(*) approach from above:

insert into Item(ItemCatalogID, Name, Description, OtherValue) values (100, 'MyItem', 'Short Description', (select count(*) from Item where ID=100));

It might be that you'll have to alias one of the occurrences of your table:

insert into Item(ItemCatalogID, Name, Description, OtherValue) values (100, 'MyItem', 'Short Description', (select count(*) from Item AS I where ID=100))

This executes in one step and you won't have to worry about a race condition.

If you cannot change this due to whatever reasons, there is another solution: use table locking.

Prefix your statements with

LOCK TABLES Counters WRITE, Item WRITE

and suffix them with

UNLOCK TABLES

in order to have exclusive write access to them.

Handling race conditions in PostgreSQL

You have to care about the transaction isolation level. It should be set to "SERIALIZABLE".

The reason are Phantom Reads - The transaction doesn't lock the whole table, but only the rows which have already been read by the transaction.

So, if another transaction inserts new data, they haven't been locked yet, and the error appears.

Serializable avoids this, by blocking all other transactions, until this one finished.

You can do this via

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

The documentations: http://www.postgresql.org/docs/9.1/static/transaction-iso.html

If you want to learn more about this topic, I can highly recommend you this video: http://www.youtube.com/watch?v=zz-Xbqp0g0A

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