EXPLAIN QUERY PLAN

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EXPLAIN QUERY PLAN
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# 1. The EXPLAIN QUERY PLAN Command

Warning: The data returned by the EXPLAIN QUERY PLAN command is intended for interactive debugging only. It may change dramatically between SQLite releases. Applications should not depend on the results of an EXPLAIN QUERY PLAN command.

The EXPLAIN QUERY PLAN SQL command is used to obtain a high-level description of the strategy or plan that SQLite uses to implement a specific SQL query. Most significantly, it reports on the way in which the query uses database indices. In interpreting and using this information to optimize database schemas and queries, users might find the documents describing how SQLite plans and optimizes queries useful.

An EXPLAIN QUERY PLAN command returns zero or more rows of four columns each. The column names are "selectid", "order", "from", "detail". The first three columns contain an integer value. The final column, "detail", contains a text value which carries most of the useful information.

EXPLAIN QUERY PLAN is most useful on a SELECT statement, but may also be appear with other statements that read data from database tables (e.g. UPDATE, DELETE, INSERT INTO ... SELECT).

## 1.1. Table and Index Scans

When processing a SELECT (or other) statement, SQLite may retrieve data from database tables in a variety of ways. It may scan through all the records in a table (a full-table scan), scan a contiguous subset of the records in a table based on the rowid index, scan a contiguous subset of the entries in a database index, or use a combination of the above strategies in a single scan. The various ways in which SQLite may retrieve data from a table or index are described in detail here.

For each table read by the query, the output of EXPLAIN QUERY PLAN includes a record for which the value in the "detail" column begins with either "SCAN" or "SEARCH". "SCAN" is used for a full-table scan, including cases where SQLite iterates through all records in a table in an order defined by an index. "SEARCH" indicates that only a subset of the table rows are visited. Each SCAN or SEARCH record includes the following information:

- The name of the table data is read from.
- Whether or not an index or automatic index is used.
- Whether or not the covering index optimization applies.
- Which terms of the WHERE clause are used for indexing.
- The estimated number of rows that SQLite will visit.

For example, the following EXPLAIN QUERY PLAN command operates on a SELECT statement that is implemented by performing a full-table scan on table t1:

    sqlite> EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1;
    0|0|0|SCAN TABLE t1 (~100000 rows)

The example above shows SQLite estimating that the full-table scan will visit approximately 1,000,000 records. If the query were able to use an index, then the SCAN/SEARCH record would include the name of the index and, for a SEARCH record, an indication of how the subset of rows visited is identified. For example:

    sqlite> CREATE INDEX i1 ON t1(a);
    sqlite> EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1;
    0|0|0|SEARCH TABLE t1 USING INDEX i1 (a=?) (~10 rows)

The previous example, SQLite uses index "i1" to optimize a WHERE clause term of the form (a=?) - in this case "a=1". SQLite estimates that about 10 records will match the "a=1" term. The previous example could not use a covering index, but the following example can, and that fact is reflected in the output:

    sqlite> CREATE INDEX i2 ON t1(a, b);
    sqlite> EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1;
    0|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=?) (~10 rows)

All joins in SQLite are implemented using nested scans. When a SELECT query that features a join is analyzed using EXPLAIN QUERY PLAN, one SCAN or SEARCH record is output for each nested loop. For example:

    sqlite> EXPLAIN QUERY PLAN SELECT t1.*, t2.* FROM t1, t2 WHERE t1.a=1 AND t1.b>2;
    0|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=? AND b>?) (~3 rows)
    0|1|1|SCAN TABLE t2 (~1000000 rows)

The second column of output (column "order") indicates the nesting order. In this case, the scan of table t1 using index i2 is the outer loop (order=0) and the full-table scan of table t2 (order=1) is the inner loop. The third column (column "from"), indicates the position in the FROM clause of the SELECT statement that the table associated with each scan occurs in. In the case above, table t1 occupies the first position in the FROM clause, so the value of column "from" is 0 in the first record. Table t2 is in the second position, so the "from" column for the corresponding SCAN record is set to 1. In the following example, the positions of t1 and t2 in the FROM clause of the SELECT are reversed. The query strategy remains the same, but the values in the "from" column of the output are adjusted accordingly.

    sqlite> EXPLAIN QUERY PLAN SELECT t1.*, t2.* FROM t2, t1 WHERE t1.a=1 AND t1.b>2;
    0|0|1|SEARCH TABLE t1 USING COVERING INDEX i2 (a=? AND b>?) (~3 rows)
    0|1|0|SCAN TABLE t2 (~1000000 rows)

In the example above, SQLite estimates that the outer loop scan will visit approximately 3 rows, and that the inner loop will visit approximately 1,000,000. If you observe that SQLite's estimates are wildly inaccurate (and appear to be causing it to generate sub-optimal query plans), your queries may benefit from running the ANALYZE command on the database.

If the WHERE clause of a query contains an OR expression, then SQLite might use the "OR by union" strategy (also described here). In this case there will be two SEARCH records, one for each index, with the same values in both the "order" and "from" columns. For example:

    sqlite> CREATE INDEX i3 ON t1(b);
    sqlite> EXPLAIN QUERY PLAN SELECT * FROM t1 WHERE a=1 OR b=2;
    0|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=?) (~10 rows)
    0|0|0|SEARCH TABLE t1 USING INDEX i3 (b=?) (~10 rows)

## 1.2. Temporary Sorting B-Trees

If a SELECT query contains an ORDER BY, GROUP BY or DISTINCT clause, SQLite may need to use a temporary b-tree structure to sort the output rows. Or, it might use an index. Using an index is almost always much more efficient than performing a sort. If a temporary b-tree is required, a record is added to the EXPLAIN QUERY PLAN output with the "detail" field set to a string value of the form "USE TEMP B-TREE FOR xxx", where xxx is one of "ORDER BY", "GROUP BY" or "DISTINCT". For example:

    sqlite> EXPLAIN QUERY PLAN SELECT c, d FROM t2 ORDER BY c;
    0|0|0|SCAN TABLE t2 (~1000000 rows)
    0|0|0|USE TEMP B-TREE FOR ORDER BY

In this case using the temporary b-tree can be avoided by creating an index on t2(c), as follows:

    sqlite> CREATE INDEX i4 ON t2(c);
    sqlite> EXPLAIN QUERY PLAN SELECT c, d FROM t2 ORDER BY c;
    0|0|0|SCAN TABLE t2 USING INDEX i4 (~1000000 rows)

## 1.3. Subqueries

In all the examples above, the first column (column "selectid") is always set to 0. If a query contains sub-selects, either as part of the FROM clause or as part of SQL expressions, then the output of EXPLAIN QUERY PLAN also includes a report for each sub-select. Each sub-select is assigned a distinct, non-zero "selectid" value. The top-level SELECT statement is always assigned the selectid value 0. For example:

    sqlite> EXPLAIN QUERY PLAN SELECT (SELECT b FROM t1 WHERE a=0), (SELECT a FROM t1 WHERE b=t2.c) FROM t2;
    0|0|0|SCAN TABLE t2 (~1000000 rows)
    0|0|0|EXECUTE SCALAR SUBQUERY 1
    1|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=?) (~10 rows)
    0|0|0|EXECUTE CORRELATED SCALAR SUBQUERY 2
    2|0|0|SEARCH TABLE t1 USING INDEX i3 (b=?) (~10 rows)

The example above contains a pair of scalar subqueries assigned selectid values 1 and 2. As well as a SCAN record, there are also 2 "EXECUTE" records associated with the top level subquery (selectid 0), indicating that subqueries 1 and 2 are executed by the top level query in a scalar context. The CORRELATED qualifier present in the EXECUTE record associated with scalar subquery 2 indicates that the query must be run separately for each row visited by the top level query. Its absence in the record associated with subquery 1 means that the subquery is only run once and the result cached. In other words, subquery 2 may be more performance critical, as it may be run many times whereas subquery 1 is only ever run once.

Unless the flattening optimization is applied, if a subquery appears in the FROM clause of a SELECT statement, SQLite executes the subquery and stores the results in a temporary table. It then uses the contents of the temporary table in place of the subquery to execute the parent query. This is shown in the output of EXPLAIN QUERY PLAN by substituting a "SCAN SUBQUERY" record for the "SCAN TABLE" record that normally appears for each element in the FROM clause. For example:

    sqlite> EXPLAIN QUERY PLAN SELECT count(*) FROM (SELECT max(b) AS x FROM t1 GROUP BY a) GROUP BY x;
    1|0|0|SCAN TABLE t1 USING COVERING INDEX i2 (~1000000 rows)
    0|0|0|SCAN SUBQUERY 1 (~1000000 rows)
    0|0|0|USE TEMP B-TREE FOR GROUP BY

If the flattening optimization is used on a subquery in the FROM clause of a SELECT statement, then the output of EXPLAIN QUERY PLAN reflects this. For example, in the following there is no "SCAN SUBQUERY" record even though there is a subquery in the FROM clause of the top level SELECT. Instead, since the flattening optimization does apply in this case, the EXPLAIN QUERY PLAN report shows that the top level query is implemented using a nested loop join of tables t1 and t2.

    sqlite> EXPLAIN QUERY PLAN SELECT * FROM (SELECT * FROM t2 WHERE c=1), t1;
    0|0|0|SEARCH TABLE t2 USING INDEX i4 (c=?) (~10 rows)
    0|1|1|SCAN TABLE t1 (~1000000 rows)

## 1.4. Compound Queries

Each component query of a compound query (UNION, UNION ALL, EXCEPT or INTERSECT) is assigned its own selectid and reported on separately. A single record is output for the parent (compound query) identifying the operation, and whether or not a temporary b-tree is used to implement it. For example:

    sqlite> EXPLAIN QUERY PLAN SELECT a FROM t1 UNION SELECT c FROM t2;
    1|0|0|SCAN TABLE t1 (~1000000 rows)
    2|0|0|SCAN TABLE t2 (~1000000 rows)
    0|0|0|COMPOUND SUBQUERIES 1 AND 2 USING TEMP B-TREE (UNION)

The "USING TEMP B-TREE" clause in the above output indicates that a temporary b-tree structure is used to implement the UNION of the results of the two sub-selects. If the temporary b-tree were not required, as in the following example, the clause is not present.

    sqlite> EXPLAIN QUERY PLAN SELECT a FROM t1 EXCEPT SELECT d FROM t2 ORDER BY 1;
    1|0|0|SCAN TABLE t1 USING COVERING INDEX i2 (~1000000 rows)
    2|0|0|SCAN TABLE t2 (~1000000 rows)
    2|0|0|USE TEMP B-TREE FOR ORDER BY
    0|0|0|COMPOUND SUBQUERIES 1 AND 2 (EXCEPT)

# 2. Sample Code

Sometimes, within a large application, it may be inconvenient to modify code to generate EXPLAIN QUERY PLAN commands for the SELECT queries being investigated. From within an interactive debugging session, it may be almost impossible. In these situations, a function similar to the following may be useful. This particular function is passed an SQLite statement handle as an argument and outputs the corresponding EXPLAIN QUERY PLAN report to standard output. Application specific versions may output the report to an application log or similar.

    /*
    ** Argument pStmt is a prepared SQL statement. This function compiles
    ** an EXPLAIN QUERY PLAN command to report on the prepared statement,
    ** and prints the report to stdout using printf().
    */
    int printExplainQueryPlan(sqlite3_stmt *pStmt){
      const char *zSql;               /* Input SQL */
      char *zExplain;                 /* SQL with EXPLAIN QUERY PLAN prepended */
      sqlite3_stmt *pExplain;         /* Compiled EXPLAIN QUERY PLAN command */
      int rc;                         /* Return code from sqlite3_prepare_v2() */

      zSql = sqlite3_sql(pStmt);
      if( zSql==0 ) return SQLITE_ERROR;

      zExplain = sqlite3_mprintf("EXPLAIN QUERY PLAN %s", zSql);
      if( zExplain==0 ) return SQLITE_NOMEM;

      rc = sqlite3_prepare_v2(sqlite3_db_handle(pStmt), zExplain, -1, &pExplain, 0);
      sqlite3_free(zExplain);
      if( rc!=SQLITE_OK ) return rc;

      while( SQLITE_ROW==sqlite3_step(pExplain) ){
        int iSelectid = sqlite3_column_int(pExplain, 0);
        int iOrder = sqlite3_column_int(pExplain, 1);
        int iFrom = sqlite3_column_int(pExplain, 2);
        const char *zDetail = (const char *)sqlite3_column_text(pExplain, 3);

        printf("%d %d %d %s\n", iSelectid, iOrder, iFrom, zDetail);
      }

      return sqlite3_finalize(pExplain);
    }