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EXPLAIN: Inspect Query Plans
EXPLAIN SELECT * FROM tbl;

The EXPLAIN statement displays the physical plan, i.e., the query plan that will get executed, and is enabled by prepending the query with EXPLAIN. The physical plan is a tree of operators that are executed in a specific order to produce the result of the query. To generate an efficient physical plan, the query optimizer transforms the existing physical plan into a better physical plan.

To demonstrate, see the below example:

CREATE TABLE students (name VARCHAR, sid INTEGER);
CREATE TABLE exams (eid INTEGER, subject VARCHAR, sid INTEGER);
INSERT INTO students VALUES ('Mark', 1), ('Joe', 2), ('Matthew', 3);
INSERT INTO exams VALUES (10, 'Physics', 1), (20, 'Chemistry', 2), (30, 'Literature', 3);

EXPLAIN ANALYZE
    SELECT name
    FROM students
    JOIN exams USING (sid)
    WHERE name LIKE 'Ma%';

┌─────────────────────────────┐
│┌───────────────────────────┐│
││       Physical Plan       ││
│└───────────────────────────┘│
└─────────────────────────────┘
┌───────────────────────────┐
│         PROJECTION        │
│   ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─   │
│            name           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         HASH_JOIN         │
│   ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─   │
│           INNER           │
│         sid = sid         ├──────────────┐
│   ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─   │              │
│           EC: 1           │              │
└─────────────┬─────────────┘              │
┌─────────────┴─────────────┐┌─────────────┴─────────────┐
│         SEQ_SCAN          ││           FILTER          │
│   ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─   ││   ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─   │
│           exams           ││     prefix(name, 'Ma')    │
│   ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─   ││   ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─   │
│            sid            ││           EC: 1           │
│   ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─   ││                           │
│           EC: 3           ││                           │
└───────────────────────────┘└─────────────┬─────────────┘
                             ┌─────────────┴─────────────┐
                             │         SEQ_SCAN          │
                             │   ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─   │
                             │          students         │
                             │   ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─   │
                             │            sid            │
                             │            name           │
                             │   ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─   │
                             │ Filters: name>=Ma AND name│
                             │  <Mb AND name IS NOT NULL │
                             │   ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─   │
                             │           EC: 1           │
                             └───────────────────────────┘

Note that the query is not actually executed – therefore, we can only see the estimated cardinality (EC) for each operator, which is calculated by using the statistics of the base tables and applying heuristics for each operator.

Additional Explain Settings

The EXPLAIN statement supports additional settings that can be used to control the output. The following settings are available:

The default setting. Only shows the physical plan.

PRAGMA explain_output = 'physical_only';

Shows only the optimized plan.

PRAGMA explain_output = 'optimized_only';

Shows both the physical and optimized plans.

PRAGMA explain_output = 'all';

See Also

For more information, see the ”Profiling” page.

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