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Comment:Move the examples on lang_explain.html to new document eqp.html.
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SHA1: 0c8678b577986758037af8ee13e3720f3fcfa3eb
User & Date: dan 2010-11-13 19:00:19
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2010-11-15
13:15
Fix a typo from the fts3.html document. check-in: c547f4b6c1 user: drh tags: trunk
2010-11-13
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Move the examples on lang_explain.html to new document eqp.html. check-in: 0c8678b577 user: dan tags: trunk
17:10
Update EXPLAIN QUERY PLAN examples to match changes to sqlite. check-in: 3f8c0a0c1f user: dan tags: trunk
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<tcl>hd_keywords {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. It may change dramatically between
SQLite releases. Applications should not depend on the results of an 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, 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 [indexing|plans] and [optimizer|optimizes] queries useful.

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

<p>EXPLAIN QUERY PLAN is most useful when used with a SELECT statement, but may
also be used on 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 the query reads data from, 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> The selectivity of the subset of records scanned.
  <li> The estimated number of rows that SQLite expects the scan to visit.
</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&gt; EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1; 
    0|0|0|SCAN TABLE t1 (~100000 rows)
</codeblock>)^

<p>
  SQLite estimates 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:
^(<codeblock>
    sqlite&gt; CREATE INDEX i1 ON t1(a);
    sqlite&gt; EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1;
    0|0|0|SEARCH TABLE t1 USING INDEX i1 (a=?) (~10 rows)
</codeblock>)^

<p>
  The output above shows that in this case, SQLite uses index "i1" to optimize
  a WHERE clause filter of the form (a=?) - in this case "a=1". SQLite 
  estimates that scanning the subset of index entries that match the "a=1"
  filter means scanning through approximately 10 records. In this case it is
  not possible to use index i1 as a [covering index], but if it were, the
  SCAN or SEARCH record would report that as well. For example:
^(<codeblock>
    sqlite&gt; CREATE INDEX i2 ON t1(a, b);
    sqlite&gt; EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1; 
    0|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=?) (~10 rows)
</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&gt; 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)
</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&gt; 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)
</codeblock>)^

<p>
  In the example above, SQLite estimates that the outer loop scan will visit
  approximately 3 rows, and the inner loop scan 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.

<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&gt; CREATE INDEX i3 ON t1(b);
    sqlite&gt; 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)
</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 perform an 
  <a href="http://en.wikipedia.org/wiki/Insertion_sort">insertion sort</a> 
  of the output rows. Or, it may [sorting|use an index]. Using an index is 
  almost always much more efficient than performing an online insertion 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&gt; 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
</codeblock>)^

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

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

<h1>Sample Code</h1>


<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){
  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 = sqlite3_column_text(pExplain, 3);

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

  return sqlite3_finalize(pExplain);
}
</codeblock>


Changes to pages/fancyformat.tcl.

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  # this text. The "<div class=startsearch>" tag tells the script that 
  # builds the site-search database not to index any text that occurs
  # before it. This stops the table of contents from being used for 
  # snippets on search results pages.
  #
  set toc [subst {
    <div class=fancy>
    <div style="font-size:2em;text-align:center;color:#80a796">
      $::Addtoc(title)
    </div>
    <div style="font-size:1.5em;margin:1em;color:#80a796">
      Table Of Contents</div>
    <div id=toc> $::Addtoc(toc) </div>
    <div class=startsearch></div>
  }]

  string map [list <table_of_contents> $toc] $::Addtoc(doc)
}








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  # this text. The "<div class=startsearch>" tag tells the script that 
  # builds the site-search database not to index any text that occurs
  # before it. This stops the table of contents from being used for 
  # snippets on search results pages.
  #
  set toc [subst {
    <div class=fancy>
    <div style="font-size:2em;text-align:center;color:#044a64">
      $::Addtoc(title)
    </div>
    <div style="font-size:1.5em;margin:1em;color:#044a64">
      Table Of Contents</div>
    <div id=toc> $::Addtoc(toc) </div>
    <div class=startsearch></div>
  }]

  string map [list <table_of_contents> $toc] $::Addtoc(doc)
}

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<p>^When the EXPLAIN keyword appears by itself it causes the statement
to behave as a query that returns the sequence of 
[virtual machine instructions] it would have used to execute the command had
the EXPLAIN keyword not been present. ^When the EXPLAIN QUERY PLAN phrase
appears, the statement returns high-level information regarding the query
plan that would have been used.

<h3>EXPLAIN QUERY PLAN Details</h3>

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

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

<h4>Table and Index Scans</h4>

<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 the query reads data from, 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> The selectivity of the subset of records scanned.
  <li> The estimated number of rows that SQLite expects the scan to visit.
</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:
<pre>^(
    sqlite&gt; EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1; 
    0|0|0|SCAN TABLE t1 (~100000 rows)
)^</pre>

<p>
  SQLite estimates 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:
<pre>^(
    sqlite&gt; CREATE INDEX i1 ON t1(a);
    sqlite&gt; EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1;
    0|0|0|SEARCH TABLE t1 USING INDEX i1 (a=?) (~10 rows)
)^</pre>

<p>
  The output above shows that in this case, SQLite uses index "i1" to optimize
  a WHERE clause filter of the form (a=?) - in this case "a=1". SQLite 
  estimates that scanning the subset of index entries that match the "a=1"
  filter means scanning through approximately 10 records. In this case it is
  not possible to use index i1 as a [covering index], but if it were, the
  SCAN or SEARCH record would report that as well. For example:
<pre>^(
    sqlite&gt; CREATE INDEX i2 ON t1(a, b);
    sqlite&gt; EXPLAIN QUERY PLAN SELECT a, b FROM t1 WHERE a=1; 
    0|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=?) (~10 rows)
)^</pre>

<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:
<pre>^(
    sqlite&gt; 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)
)^</pre>

<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.
<pre>^(
    sqlite&gt; 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)
)^</pre>

<p>
  In the example above, SQLite estimates that the outer loop scan will visit
  approximately 3 rows, and the inner loop scan 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.

<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: 
<pre>^(
    sqlite&gt; CREATE INDEX i3 ON t1(b);
    sqlite&gt; 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)
)^</pre>

<h4>Temporary Sorting B-Trees</h4>

<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 perform an 
  <a href="http://en.wikipedia.org/wiki/Insertion_sort">insertion sort</a> 
  of the output rows. Or, it may [sorting|use an index]. Using an index is 
  almost always much more efficient than performing an online insertion 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:

<pre>^(
    sqlite&gt; 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
)^</pre>

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

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

<h4>Subqueries</h4>

<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:

<pre>^(
    sqlite&gt; 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)
)^</pre>

<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:

<pre>^(
    sqlite&gt; 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
)^</pre>

<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.

<pre>^(
    sqlite&gt; 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)
)^</pre>

<h4>Compound Queries</h4>

<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:

<pre>^(
    sqlite&gt; 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)
)^</pre>

<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.

<pre>^(
    sqlite&gt; 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)
)^</pre>



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<p>^When the EXPLAIN keyword appears by itself it causes the statement
to behave as a query that returns the sequence of 
[virtual machine instructions] it would have used to execute the command had
the EXPLAIN keyword not been present. ^When the EXPLAIN QUERY PLAN phrase
appears, the statement returns high-level information regarding the query
plan that would have been used.

The EXPLAIN QUERY PLAN command is described in 


















































































































































































































































[explain query plan|more detail here].

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