<tcl>hd_keywords {explain query plan} {EXPLAIN QUERY PLAN}</tcl>
<title>EXPLAIN QUERY PLAN</title>
<style>codeblock { display:block;white-space:pre;font-family:fixed }</style>
<table_of_contents>
<h1>The EXPLAIN QUERY PLAN Command</h1>
<p style="margin-left:10ex;margin-right:10ex">
<b>Warning: The data returned by the EXPLAIN QUERY PLAN command is
intended for interactive debugging only. The output format may change
between SQLite releases. Applications should not depend on the output
format of the EXPLAIN QUERY PLAN command.</b>
<p>The [EXPLAIN|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, EXPLAIN QUERY PLAN reports on the way in
which the query uses database indices. This document is a guide to
understanding and interpreting the EXPLAIN QUERY PLAN output. Background
information is available separately:
<ul>
<li> Notes on the [optimizer | query optimizer].
<li> How [indexing] works.
<li> The [next generation query planner].
</ul>
<p>^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.
<p>^(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).)^
<h2>Table and Index Scans</h2>
<p>
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 [CREATE TABLE|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 [strategies|here].
<p>
^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:
<ul>
<li> The name of the table data is read from.
<li> Whether or not an index or [automatic indexing|automatic index] is used.
<li> Whether or not the [covering index] optimization applies.
<li> Which terms of the WHERE clause are used for indexing.
</ul>)^
<p>
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:
^(<codeblock>
sqlite> EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1;
0|0|0|SCAN TABLE t1
</codeblock>)^
<p>
The example above shows
SQLite estimating that the full-table scan will visit approximately
100,000 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:
^(<codeblock>
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
</codeblock>)^
<p>
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:
^(<codeblock>
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=?)
</codeblock>)^
<p>
All joins in SQLite are [join order|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:
^(<codeblock>
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>?)
0|1|1|SCAN TABLE t2
</codeblock>)^
<p>
^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.
^(<codeblock>
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>?)
0|1|0|SCAN TABLE t2
</codeblock>)^
<p>
If the WHERE clause of a query contains an OR expression, then SQLite might
use the [or-connected-terms|"OR by union"] strategy (also described
[or optimization|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:
^(<codeblock>
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=?)
0|0|0|SEARCH TABLE t1 USING INDEX i3 (b=?)
</codeblock>)^
<h2>Temporary Sorting B-Trees</h2>
<p>
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 [sorting|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:
^(<codeblock>
sqlite> EXPLAIN QUERY PLAN SELECT c, d FROM t2 ORDER BY c;
0|0|0|SCAN TABLE t2
0|0|0|USE TEMP B-TREE FOR ORDER BY
</codeblock>)^
<p>
In this case using the temporary b-tree can be avoided by creating an index
on t2(c), as follows:
^(<codeblock>
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
</codeblock>)^
<h2>Subqueries</h2>
<p>
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:
^(<codeblock>
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
0|0|0|EXECUTE SCALAR SUBQUERY 1
1|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=?)
0|0|0|EXECUTE CORRELATED SCALAR SUBQUERY 2
2|0|0|SEARCH TABLE t1 USING INDEX i3 (b=?)
</codeblock>)^
<p>
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.
<p>
^(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:
^(<codeblock>
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
0|0|0|SCAN SUBQUERY 1
0|0|0|USE TEMP B-TREE FOR GROUP BY
</codeblock>)^
<p>
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.
^(<codeblock>
sqlite> EXPLAIN QUERY PLAN SELECT * FROM (SELECT * FROM t2 WHERE c=1), t1;
0|0|0|SEARCH TABLE t2 USING INDEX i4 (c=?)
0|1|1|SCAN TABLE t1
</codeblock>)^
<h2>Compound Queries</h2>
<p>
^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:
^(<codeblock>
sqlite> EXPLAIN QUERY PLAN SELECT a FROM t1 UNION SELECT c FROM t2;
1|0|0|SCAN TABLE t1
2|0|0|SCAN TABLE t2
0|0|0|COMPOUND SUBQUERIES 1 AND 2 USING TEMP B-TREE (UNION)
</codeblock>)^
<p>
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.
^(<codeblock>
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
2|0|0|SCAN TABLE t2
2|0|0|USE TEMP B-TREE FOR ORDER BY
0|0|0|COMPOUND SUBQUERIES 1 AND 2 (EXCEPT)
</codeblock>)^
<h1>Sample Code</h1>
<p>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.
<!--
** WARNING: The following function, printExplainQueryPlan() is duplicated
** in file test1.c of the source distribution for testing purposes. If
** the code below needs to be modified, the copy in test1.c should also
** be modified and the Tcl tests run.
-->
<codeblock>
/*
** 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);
}
</codeblock>