Is it possible to produce phantom read in single SQL Server query? - sql-server

Question

All of explanations of phantom reads I managed to find demonstrate phantom read by running 2 select statements in one transaction (e.g. https://blobeater.blog/2017/10/26/sql-server-phantom-reads/ )
BEGIN TRAN
SELECT #1
DELAY DURING WHICH AN INSERT TAKES PLACE IN A DIFFERENT TRANSACTION
SELECT #2
END TRAN
Is it possible to reproduce a phantom read in one select statement? This would mean that select statement starts on transaction #1. Then insert runs on transaction #2 and commits. Finally select statement from transaction #1 completes, but does not return a row that transaction #2 has inserted.

Answer 1

The SQL Server Transaction Isolation Levels documentation defines a phantom row as one "that matches the search criteria but is not initially seen" (emphasis mine). Consequently, more than one SELECT statement is needed for a phantom read to occur.
Data inserted during execution SELECT statement execution might not be returned in the READ COMMITTED isolation level depending on the timing but this is not a phantom read by definition. The example below shows this behavior.
--create table with enough data for a long-running SELECT query
CREATE TABLE dbo.PhantomReadExample(
PhantomReadExampleID int NOT NULL
CONSTRAINT PK_PhantomReadExample PRIMARY KEY
, PhantomReadData char(8000) NOT NULL
);
--insert 100K rows
WITH
t10 AS (SELECT n FROM (VALUES(0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) t(n))
,t1k AS (SELECT 0 AS n FROM t10 AS a CROSS JOIN t10 AS b CROSS JOIN t10 AS c)
,t1m AS (SELECT ROW_NUMBER() OVER (ORDER BY (SELECT 0)) AS num FROM t1k AS a CROSS JOIN t1k AS b)
INSERT INTO dbo.PhantomReadExample WITH(TABLOCKX) (PhantomReadExampleID, PhantomReadData)
SELECT num*2, 'data'
FROM t1m
WHERE num <= 100000;
GO
--run this on connection 1
SELECT *
FROM dbo.PhantomReadExample
ORDER BY PhantomReadExampleID;
GO
--run this on connection 2 while the connection 1 SELECT is running
INSERT INTO dbo.PhantomReadExample(PhantomReadExampleID, PhantomReadData)
VALUES(1, 'data');
GO
Shared locks are acquired on rows as they are read during the SELECT query scan to ensure only committed data are read but these are immediately released once data are read improve concurrency. This allows other sessions to insert, update, and delete rows while the SELECT query is running.
The inserted row is not returned in this case because the ordered clustered index scan had already past the point of the insert.

Answer 2

Below is the wikipedia definition of phantom reads
A phantom read occurs when, in the course of a transaction, new rows
are added by another transaction to the records being read.
This can occur when range locks are not acquired on performing a
SELECT ... WHERE operation. The phantom reads anomaly is a special
case of Non-repeatable reads when Transaction 1 repeats a ranged
SELECT ... WHERE query and, between both operations, Transaction 2
creates (i.e. INSERT) new rows (in the target table) which fulfill
that WHERE clause.
This is certainly possible to reproduce in a single reading query (of course other database activity must also be happening to produce the phantom rows).
Setup
CREATE TABLE Test(X INT PRIMARY KEY);
Connection 1 (leave this running)
SET NOCOUNT ON;
WHILE 1 = 1
INSERT INTO Test VALUES (CRYPT_GEN_RANDOM(4))
Connection 2
This is extremely likely to return some rows if running at read committed lock isolation level (default for the on premise product and enforced with table hint below)
WITH CTE AS
(
SELECT *
FROM Test WITH (READCOMMITTEDLOCK)
WHERE X BETWEEN 0 AND 2147483647
)
SELECT *
FROM CTE c1
FULL OUTER HASH JOIN CTE c2 ON c1.X = c2.X
WHERE (c1.X IS NULL OR c2.X IS NULL)
The returned rows are values added between the first and second read of the table for rows matching the WHERE X BETWEEN 0 AND 2147483647 predicate.