Short presentation for "Testing Database Engines via Pivoted Query Synthesis" at OSDI '20

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Testing Database Engines via Pivoted Query Synthesis Manuel Rigger Zhendong Su ETH Zurich, Switzerland 11/05/2020 @RiggerManuel @ast_eth https://people.inf.ethz.ch/suz/

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2 Database Management Systems (DBMSs) PostgreSQL

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3 Database Management Systems (DBMSs) PostgreSQL “it is seems likely that there are over one trillion (1e12) SQLite databases in active use” https://www.sqlite.org/mostdeployed.html

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4 Database Management Systems (DBMSs) PostgreSQL We found 96 unique bugs in these DBMSs, 78 of which were fixed!

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5 Goal: Find Logic Bugs Logic bugs: DBMS returns an incorrect result set

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6 Example: SQLite3 Bug c0 0 1 2 NULL t0 CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; https://sqlite.org/src/tktview/80256748471a01

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7 Example: SQLite3 Bug c0 0 1 2 NULL t0 CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; IS NOT is a “null-safe” comparison operator https://sqlite.org/src/tktview/80256748471a01

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8 Example: SQLite3 Bug c0 0 1 2 NULL t0 CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; https://sqlite.org/src/tktview/80256748471a01

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9 Example: SQLite3 Bug c0 0 1 2 NULL t0 0 CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; TRUE https://sqlite.org/src/tktview/80256748471a01

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10 Example: SQLite3 Bug c0 0 1 2 NULL t0 0 CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; TRUE 0 https://sqlite.org/src/tktview/80256748471a01

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11 Example: SQLite3 Bug c0 0 1 2 NULL t0 1 CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; 0 FALSE https://sqlite.org/src/tktview/80256748471a01

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12 Example: SQLite3 Bug c0 0 1 2 NULL t0 0 2 2 CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; TRUE https://sqlite.org/src/tktview/80256748471a01

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13 Example: SQLite3 Bug c0 0 1 2 NULL t0 0 2 NULL NULL CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; TRUE https://sqlite.org/src/tktview/80256748471a01

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14 Example: SQLite3 Bug c0 0 1 2 NULL t0 NULL CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; TRUE 0 2 NULL https://sqlite.org/src/tktview/80256748471a01

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15 Example: SQLite3 Bug c0 0 1 2 NULL t0 NULL CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; TRUE 0 2 NULL 0 2 https://sqlite.org/src/tktview/80256748471a01

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16 Example: SQLite3 Bug c0 0 1 2 NULL t0 NULL CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; TRUE  NULL was not contained in the result set! 0 2 NULL 0 2 https://sqlite.org/src/tktview/80256748471a01

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17 Background: Differential Testing PostgreSQL SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; Massive Stochastic Testing of SQL by Slutz, 1998.

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18 Background: Differential Testing PostgreSQL RS1 RS2 RS3 SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; Massive Stochastic Testing of SQL by Slutz, 1998.

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19 Background: Differential Testing PostgreSQL RS1 RS2 RS3 SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; Check that all DBMSs compute the same result (RS1 = RS2 = RS3 ) Massive Stochastic Testing of SQL by Slutz, 1998.

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20 Background: Differential Testing PostgreSQL RS1 RS2 RS3

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21 Background: Differential Testing PostgreSQL RS1 RS2 RS3

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22 Background: Differential Testing {0, 2} Syntax error Syntax error PostgreSQL

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23 Background: Differential Testing CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (3), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1;

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24 Background: Differential Testing CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (3), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; MySQL and PostgreSQL require a data type definition

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25 Background: Differential Testing CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (3), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; PostgreSQL provides an IS DISTINCT FROM operator, and MySQL a <=> null-safe comparison operator

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26 Idea: PQS Pivoted Query Synthesis (PQS): Divide-and-conquer approach for testing DBMSs

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27 PQS Idea c0 0 1 2 NULL t0 CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (3), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; Validate the result set based on one randomly-selected row

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28 PQS Idea c0 0 1 2 NULL t0 CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (3), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; Pivot row Validate the result set based on one randomly-selected row

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29 PQS Idea c0 0 1 2 NULL t0 CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (3), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1; Generate a query that is guaranteed to at least fetch the pivot row NULL TRUE

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30 PQS Idea c0 0 1 2 NULL t0 CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (3), (NULL); SELECT c0 FROM t0 WHERE t0.c0 IS NOT 1;  If the pivot row is missing from the result set a bug has been detected 0 2

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31 Approach Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained

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32 Approach Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained CREATE TABLE t0(c0); CREATE INDEX i0 ON t0(1) WHERE c0 NOT NULL; INSERT INTO t0 (c0) VALUES (0), (1), (2), (3), (NULL); Statements are heuristically generated based on the DBMS’ SQL dialect

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33 Approach Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained One random row from multiple tables and views

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34 Approach Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained Generate predicatesthat evaluate to TRUE for the pivot row and use them in JOIN and WHERE clauses SELECT c0 FROM t0 WHERE

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35 Random Expression Generation t0.c0 IS NOT 1; Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained IS NOT t0.c0 1

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36 Random Expression Generation t0.c0 IS NOT 1; We implemented an expression evaluator for each node Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained IS NOT t0.c0 1

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37 Random Expression Generation c0 0 1 2 NULL t0 Evaluate the tree based on the pivot row Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained IS NOT t0.c0 1

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38 Random Expression Generation Column references return the values from the pivot row c0 0 1 2 NULL t0 Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained IS NOT t0.c0 1

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39 Random Expression Generation Column references return the values from the pivot row c0 0 1 2 NULL t0 Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained IS NOT t0.c0 1 NULL

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40 Random Expression Generation Constant nodes return their assigned literal values c0 0 1 2 NULL t0 Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained IS NOT t0.c0 1 NULL

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41 Random Expression Generation Constant nodes return their assigned literal values c0 0 1 2 NULL t0 Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained IS NOT t0.c0 1 NULL 1

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42 Random Expression Generation Compound nodes compute their result based on their children TRUE c0 0 1 2 NULL t0 Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained IS NOT t0.c0 1 NULL 1

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43 Random Expression Generation Compound nodes compute their result based on their children TRUE c0 0 1 2 NULL t0 Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained IS NOT t0.c0 1 NULL 1 TRUE

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44 t0.c0 IS NOT 1; Query Synthesis SELECT c0 c0 FROM t0 WHERE Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained

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45 t0.c0 IS NOT 1; Query Synthesis SELECT c0 c0 FROM t0 WHERE What if the expression does not evaluate to TRUE? Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained

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46 Random Expression Rectification switch (result) { case TRUE: result = randexpr; case FALSE: result = NOT randexpr; case NULL: result = randexpr IS NULL; } Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained

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47 Random Expression Rectification switch (result) { case TRUE: result = randexpr; case FALSE: result = NOT randexpr; case NULL: result = randexpr IS NULL; } Alternatively, we could validate that the pivot row is expectedly not fetched Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained

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48 Approach SELECT (NULL) INTERSECT SELECT c0 FROM t0 WHERE NULL IS NOT 1; Rely on the DBMS to check whether the row is contained Randomly generate database Select pivot row Generate query for the pivot row Validate that the pivot row is contained

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49 Implementation https://github.com/sqlancer

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50 Bugs Overview DBMS Fixed Verified SQLite 64 0 MySQL 17 7 PostgreSQL 5 3

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51 Bugs Overview DBMS Fixed Verified SQLite 64 0 MySQL 17 7 PostgreSQL 5 3 96 bugs were unique, previously unknown ones

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52 Oracles DBMS Logic Error Crash SQLite 46 17 2 MySQL 14 10 1 PostgreSQL 1 7 1 61 were logic bugs

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53 Discussion: Limitations • Implementation effort for complex operations • Requires understanding of the SQL semantics • Aggregate and window functions • Ordering • Duplicate rows

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54 Discussion: Bug Importance https://www.mail-archive.com/[email protected]/msg117440.html CREATE TABLE t0 (c0); CREATE TABLE t1 (c1); INSERT INTO t0 VALUES (1); SELECT c0 FROM t0 LEFT JOIN t1 ON c1=c0 WHERE NOT (c1 IS NOT NULL AND c1=2);

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55 Discussion: Bug Importance This is a cut-down example, right ? You can't possibly mean to do that WHERE clause in production code. https://www.mail-archive.com/[email protected]/msg117440.html CREATE TABLE t0 (c0); CREATE TABLE t1 (c1); INSERT INTO t0 VALUES (1); SELECT c0 FROM t0 LEFT JOIN t1 ON c1=c0 WHERE NOT (c1 IS NOT NULL AND c1=2);

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56 Discussion: Bug Importance I might not spell it like that myself, but a code generator would do it (and much worse!). This example was simplified from a query generated by a Django ORM queryset using .exclude(nullable_joined_table__column=1), for instance. This is a cut-down example, right ? You can't possibly mean to do that WHERE clause in production code. https://www.mail-archive.com/[email protected]/msg117440.html CREATE TABLE t0 (c0); CREATE TABLE t1 (c1); INSERT INTO t0 VALUES (1); SELECT c0 FROM t0 LEFT JOIN t1 ON c1=c0 WHERE NOT (c1 IS NOT NULL AND c1=2);

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57 Discussion: Bug Importance I might not spell it like that myself, but a code generator would do it (and much worse!). This example was simplified from a query generated by a Django ORM queryset using .exclude(nullable_joined_table__column=1), for instance. This is a cut-down example, right ? You can't possibly mean to do that WHERE clause in production code. https://www.mail-archive.com/[email protected]/msg117440.html Even “obscure” bugs might affect users CREATE TABLE t0 (c0); CREATE TABLE t1 (c1); INSERT INTO t0 VALUES (1); SELECT c0 FROM t0 LEFT JOIN t1 ON c1=c0 WHERE NOT (c1 IS NOT NULL AND c1=2);

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58 @RiggerManuel [email protected] Summary Goal: Detect logic bugs PQS randomly selects a pivot row Rectify a random expression Evaluation: Close to 100 bugs in DBMSs