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Commit5d80171

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Back-patchfcff8a5 as a bug fix.
When there is both a serialization failure and a unique violation,throw the former rather than the latter. When initially pushed,this was viewed as a feature to assist application frameworkdevelopers, so that they could more accurately determine when toretry a failed transaction, but a test case presented by IanJackson has shown that this patch can prevent serializationanomalies in some cases where a unique violation is caught within asubtransaction, the work of that subtransaction is discarded, andno error is thrown. That makes this a bug fix, so it is beingback-patched to all supported branches where it is not alreadypresent (i.e., 9.2 to 9.5).Discussion:https://postgr.es/m/1481307991-16971-1-git-send-email-ian.jackson@eu.citrix.comDiscussion:https://postgr.es/m/22607.56276.807567.924144@mariner.uk.xensource.com
1 parent2bdee07 commit5d80171

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11 files changed

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11 files changed

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‎doc/src/sgml/mvcc.sgml

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@@ -594,7 +594,7 @@ ERROR: could not serialize access due to read/write dependencies among transact
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first. In <productname>PostgreSQL</productname> these locks do not
595595
cause any blocking and therefore can <emphasis>not</> play any part in
596596
causing a deadlock. They are used to identify and flag dependencies
597-
among concurrentserializable transactions which in certain combinations
597+
among concurrentSerializable transactions which in certain combinations
598598
can lead to serialization anomalies. In contrast, a Read Committed or
599599
Repeatable Read transaction which wants to ensure data consistency may
600600
need to take out a lock on an entire table, which could block other
@@ -629,12 +629,13 @@ ERROR: could not serialize access due to read/write dependencies among transact
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<para>
631631
Consistent use of Serializable transactions can simplify development.
632-
The guarantee that any set of concurrent serializable transactions will
633-
have the same effect as if they were run one at a time means that if
634-
you can demonstrate that a single transaction, as written, will do the
635-
right thing when run by itself, you can have confidence that it will
636-
do the right thing in any mix of serializable transactions, even without
637-
any information about what those other transactions might do. It is
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The guarantee that any set of successfully committed concurrent
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Serializable transactions will have the same effect as if they were run
634+
one at a time means that if you can demonstrate that a single transaction,
635+
as written, will do the right thing when run by itself, you can have
636+
confidence that it will do the right thing in any mix of Serializable
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transactions, even without any information about what those other
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transactions might do, or it will not successfully commit. It is
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important that an environment which uses this technique have a
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generalized way of handling serialization failures (which always return
640641
with a SQLSTATE value of '40001'), because it will be very hard to
@@ -648,6 +649,26 @@ ERROR: could not serialize access due to read/write dependencies among transact
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for some environments.
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</para>
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<para>
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While <productname>PostgreSQL</>'s Serializable transaction isolation
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level only allows concurrent transactions to commit if it can prove there
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is a serial order of execution that would produce the same effect, it
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doesn't always prevent errors from being raised that would not occur in
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true serial execution. In particular, it is possible to see unique
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constraint violations caused by conflicts with overlapping Serializable
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transactions even after explicitly checking that the key isn't present
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before attempting to insert it. This can be avoided by making sure
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that <emphasis>all</> Serializable transactions that insert potentially
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conflicting keys explicitly check if they can do so first. For example,
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imagine an application that asks the user for a new key and then checks
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that it doesn't exist already by trying to select it first, or generates
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a new key by selecting the maximum existing key and adding one. If some
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Serializable transactions insert new keys directly without following this
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protocol, unique constraints violations might be reported even in cases
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where they could not occur in a serial execution of the concurrent
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transactions.
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</para>
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<para>
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For optimal performance when relying on Serializable transactions for
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concurrency control, these issues should be considered:

‎src/backend/access/nbtree/nbtinsert.c

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@@ -369,6 +369,14 @@ _bt_check_unique(Relation rel, IndexTuple itup, Relation heapRel,
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break;
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}
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/*
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* Check for a conflict-in as we would if we were going to
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* write to this page. We aren't actually going to write,
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* but we want a chance to report SSI conflicts that would
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* otherwise be masked by this unique constraint violation.
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*/
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CheckForSerializableConflictIn(rel,NULL,buf);
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/*
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* This is a definite conflict. Break the tuple down into
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* datums and report the error. But first, make sure we
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Parsed test spec with 2 sessions
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starting permutation: r1 r2 w1 w2 c1 c2
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step r1: SELECT * FROM test WHERE i = 42;
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i
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step r2: SELECT * FROM test WHERE i = 42;
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i
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step w1: INSERT INTO test VALUES (42);
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step w2: INSERT INTO test VALUES (42); <waiting ...>
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step c1: COMMIT;
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step w2: <... completed>
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error in steps c1 w2: ERROR: could not serialize access due to read/write dependencies among transactions
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step c2: COMMIT;
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starting permutation: r1 w1 c1 r2 w2 c2
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step r1: SELECT * FROM test WHERE i = 42;
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i
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step w1: INSERT INTO test VALUES (42);
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step c1: COMMIT;
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step r2: SELECT * FROM test WHERE i = 42;
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i
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42
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step w2: INSERT INTO test VALUES (42);
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ERROR: duplicate key value violates unique constraint "test_pkey"
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step c2: COMMIT;
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@@ -0,0 +1,12 @@
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Parsed test spec with 2 sessions
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starting permutation: rw1 rw2 c1 c2
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step rw1: SELECT insert_unique(1, '1');
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insert_unique
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step rw2: SELECT insert_unique(1, '2'); <waiting ...>
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step c1: COMMIT;
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step rw2: <... completed>
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error in steps c1 rw2: ERROR: could not serialize access due to read/write dependencies among transactions
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step c2: COMMIT;
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Parsed test spec with 2 sessions
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starting permutation: r1 r2 w1 w2 c1 c2
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step r1: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
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coalesce
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3
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step r2: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
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coalesce
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3
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step w1: INSERT INTO invoice VALUES (2016, 3);
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step w2: INSERT INTO invoice VALUES (2016, 3); <waiting ...>
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step c1: COMMIT;
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step w2: <... completed>
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error in steps c1 w2: ERROR: could not serialize access due to read/write dependencies among transactions
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step c2: COMMIT;
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starting permutation: r1 w1 w2 c1 c2
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step r1: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
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coalesce
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3
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step w1: INSERT INTO invoice VALUES (2016, 3);
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step w2: INSERT INTO invoice VALUES (2016, 3); <waiting ...>
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step c1: COMMIT;
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step w2: <... completed>
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error in steps c1 w2: ERROR: duplicate key value violates unique constraint "invoice_pkey"
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step c2: COMMIT;
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starting permutation: r2 w1 w2 c1 c2
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step r2: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
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coalesce
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3
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step w1: INSERT INTO invoice VALUES (2016, 3);
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step w2: INSERT INTO invoice VALUES (2016, 3); <waiting ...>
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step c1: COMMIT;
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step w2: <... completed>
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error in steps c1 w2: ERROR: duplicate key value violates unique constraint "invoice_pkey"
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step c2: COMMIT;
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Parsed test spec with 2 sessions
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starting permutation: r1 r2 w1 w2 c1 c2
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step r1: SELECT * FROM test;
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i
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step r2: SELECT * FROM test;
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i
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step w1: INSERT INTO test VALUES (42);
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step w2: INSERT INTO test VALUES (42); <waiting ...>
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step c1: COMMIT;
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step w2: <... completed>
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error in steps c1 w2: ERROR: could not serialize access due to read/write dependencies among transactions
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step c2: COMMIT;
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starting permutation: r1 w1 c1 r2 w2 c2
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step r1: SELECT * FROM test;
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i
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step w1: INSERT INTO test VALUES (42);
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step c1: COMMIT;
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step r2: SELECT * FROM test;
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i
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42
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step w2: INSERT INTO test VALUES (42);
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ERROR: duplicate key value violates unique constraint "test_pkey"
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step c2: COMMIT;

‎src/test/isolation/isolation_schedule

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@@ -1,3 +1,7 @@
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test: read-write-unique
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test: read-write-unique-2
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test: read-write-unique-3
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test: read-write-unique-4
15
test: simple-write-skew
26
test: receipt-report
37
test: temporal-range-integrity
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# Read-write-unique test.
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setup
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{
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CREATETABLEtest(iintegerPRIMARYKEY);
6+
}
7+
8+
teardown
9+
{
10+
DROPTABLEtest;
11+
}
12+
13+
session"s1"
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setup{ BEGINISOLATIONLEVELSERIALIZABLE;}
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step"r1"{SELECT *FROMtestWHEREi=42;}
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step"w1"{INSERTINTOtestVALUES(42);}
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step"c1"{COMMIT;}
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session"s2"
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setup{ BEGINISOLATIONLEVELSERIALIZABLE;}
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step"r2"{SELECT *FROMtestWHEREi=42;}
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step"w2"{INSERTINTOtestVALUES(42);}
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step"c2"{COMMIT;}
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# Two SSI transactions see that there is no row with value 42
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# in the table, then try to insert that value; T1 inserts,
27+
# and then T2 blocks waiting for T1 to commit. Finally,
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# T2 reports a serialization failure.
29+
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permutation"r1""r2""w1""w2""c1""c2"
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# If the value is already visible before T2 begins, then a
33+
# regular unique constraint violation should still be raised
34+
# by T2.
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permutation"r1""w1""c1""r2""w2""c2"
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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,33 @@
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# Read-write-unique test.
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# From bug report 9301.
3+
4+
setup
5+
{
6+
CREATETABLEtest (
7+
keyintegerUNIQUE,
8+
valtext
9+
);
10+
11+
CREATEORREPLACEFUNCTIONinsert_unique(kinteger,vtext)RETURNSvoid
12+
LANGUAGESQLAS $$
13+
INSERTINTOtest (key,val)SELECTk,vWHERENOTEXISTS (SELECTkeyFROMtestWHEREkey=k);
14+
$$;
15+
}
16+
17+
teardown
18+
{
19+
DROPFUNCTIONinsert_unique(integer,text);
20+
DROPTABLEtest;
21+
}
22+
23+
session"s1"
24+
setup {BEGINISOLATIONLEVELSERIALIZABLE; }
25+
step"rw1" {SELECTinsert_unique(1,'1'); }
26+
step"c1" {COMMIT; }
27+
28+
session"s2"
29+
setup {BEGINISOLATIONLEVELSERIALIZABLE; }
30+
step"rw2" {SELECTinsert_unique(1,'2'); }
31+
step"c2" {COMMIT; }
32+
33+
permutation"rw1""rw2""c1""c2"
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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,48 @@
1+
# Read-write-unique test.
2+
# Implementing a gapless sequence of ID numbers for each year.
3+
4+
setup
5+
{
6+
CREATETABLEinvoice (
7+
yearint,
8+
invoice_numberint,
9+
PRIMARYKEY (year,invoice_number)
10+
);
11+
12+
INSERTINTOinvoiceVALUES (2016,1), (2016,2);
13+
}
14+
15+
teardown
16+
{
17+
DROPTABLEinvoice;
18+
}
19+
20+
session"s1"
21+
setup {BEGINISOLATIONLEVELSERIALIZABLE; }
22+
step"r1" {SELECTCOALESCE(MAX(invoice_number)+1,1)FROMinvoiceWHEREyear=2016; }
23+
step"w1" {INSERTINTOinvoiceVALUES (2016,3); }
24+
step"c1" {COMMIT; }
25+
26+
session"s2"
27+
setup {BEGINISOLATIONLEVELSERIALIZABLE; }
28+
step"r2" {SELECTCOALESCE(MAX(invoice_number)+1,1)FROMinvoiceWHEREyear=2016; }
29+
step"w2" {INSERTINTOinvoiceVALUES (2016,3); }
30+
step"c2" {COMMIT; }
31+
32+
# if they both read first then there should be an SSI conflict
33+
permutation"r1""r2""w1""w2""c1""c2"
34+
35+
# cases where one session doesn't explicitly read before writing:
36+
37+
# if s2 doesn't explicitly read, then trying to insert the value
38+
# generates a unique constraint violation after s1 commits, as if s2
39+
# ran after s1
40+
permutation"r1""w1""w2""c1""c2"
41+
42+
# if s1 doesn't explicitly read, but s2 does, then s1 inserts and
43+
# commits first, should s2 experience an SSI failure instead of a
44+
# unique constraint violation? there is no serial order of operations
45+
# (s1, s2) or (s2, s1) where s1 succeeds, and s2 doesn't see the row
46+
# in an explicit select but then fails to insert due to unique
47+
# constraint violation
48+
permutation"r2""w1""w2""c1""c2"
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1+
# Read-write-unique test.
2+
3+
setup
4+
{
5+
CREATETABLEtest(iintegerPRIMARYKEY);
6+
}
7+
8+
teardown
9+
{
10+
DROPTABLEtest;
11+
}
12+
13+
session"s1"
14+
setup{ BEGINISOLATIONLEVELSERIALIZABLE;}
15+
step"r1"{SELECT *FROMtest;}
16+
step"w1"{INSERTINTOtestVALUES(42);}
17+
step"c1"{COMMIT;}
18+
19+
session"s2"
20+
setup{ BEGINISOLATIONLEVELSERIALIZABLE;}
21+
step"r2"{SELECT *FROMtest;}
22+
step"w2"{INSERTINTOtestVALUES(42);}
23+
step"c2"{COMMIT;}
24+
25+
# Two SSI transactions see that there is no row with value 42
26+
# in the table, then try to insert that value; T1 inserts,
27+
# and then T2 blocks waiting for T1 to commit. Finally,
28+
# T2 reports a serialization failure.
29+
#
30+
# (In an earlier version of Postgres, T2 would report a unique
31+
# constraint violation).
32+
33+
permutation"r1""r2""w1""w2""c1""c2"
34+
35+
# If the value is already visible before T2 begins, then a
36+
# regular unique constraint violation should still be raised
37+
# by T2.
38+
39+
permutation"r1""w1""c1""r2""w2""c2"

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