opt_explain.cc File Reference

"EXPLAIN <command>" implementation. More...

#include "sql/opt_explain.h"
#include <sys/types.h>
#include <algorithm>
#include <atomic>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <functional>
#include <string>
#include <vector>
#include "ft_global.h"
#include "lex_string.h"
#include "m_ctype.h"
#include "m_string.h"
#include "mem_root_deque.h"
#include "my_alloc.h"
#include "my_base.h"
#include "my_bitmap.h"
#include "my_dbug.h"
#include "my_double2ulonglong.h"
#include "my_inttypes.h"
#include "my_sqlcommand.h"
#include "my_sys.h"
#include "my_thread_local.h"
#include "mysql_com.h"
#include "mysqld_error.h"
#include "sql/auth/auth_acls.h"
#include "sql/auth/sql_security_ctx.h"
#include "sql/current_thd.h"
#include "sql/debug_sync.h"
#include "sql/derror.h"
#include "sql/enum_query_type.h"
#include "sql/field.h"
#include "sql/handler.h"
#include "sql/item.h"
#include "sql/item_func.h"
#include "sql/item_subselect.h"
#include "sql/join_optimizer/access_path.h"
#include "sql/join_optimizer/bit_utils.h"
#include "sql/join_optimizer/explain_access_path.h"
#include "sql/key.h"
#include "sql/mysqld.h"
#include "sql/mysqld_thd_manager.h"
#include "sql/opt_costmodel.h"
#include "sql/opt_explain_format.h"
#include "sql/opt_trace.h"
#include "sql/parse_tree_node_base.h"
#include "sql/protocol.h"
#include "sql/query_term.h"
#include "sql/range_optimizer/group_index_skip_scan_plan.h"
#include "sql/range_optimizer/path_helpers.h"
#include "sql/sql_bitmap.h"
#include "sql/sql_class.h"
#include "sql/sql_cmd.h"
#include "sql/sql_const.h"
#include "sql/sql_error.h"
#include "sql/sql_executor.h"
#include "sql/sql_lex.h"
#include "sql/sql_list.h"
#include "sql/sql_opt_exec_shared.h"
#include "sql/sql_optimizer.h"
#include "sql/sql_parse.h"
#include "sql/sql_partition.h"
#include "sql/sql_select.h"
#include "sql/table.h"
#include "sql/table_function.h"
#include "sql/visible_fields.h"
#include "sql_string.h"
#include "template_utils.h"

Classes
class	anonymous_namespace{opt_explain.cc}::Explain
	A base for all Explain_* classes. More...
class	anonymous_namespace{opt_explain.cc}::Explain::Lazy_condition
class	anonymous_namespace{opt_explain.cc}::Explain_no_table
	Explain_no_table class outputs a trivial EXPLAIN row with "extra" column. More...
class	anonymous_namespace{opt_explain.cc}::Explain_setop_result
	Explain_union_result class outputs EXPLAIN row for UNION. More...
class	anonymous_namespace{opt_explain.cc}::Explain_table_base
	Common base class for Explain_join and Explain_table. More...
class	anonymous_namespace{opt_explain.cc}::Explain_join
	Explain_join class produces EXPLAIN output for JOINs. More...
class	anonymous_namespace{opt_explain.cc}::Explain_table
	Explain_table class produce EXPLAIN output for queries without top-level JOIN. More...
class	anonymous_namespace{opt_explain.cc}::Explain_secondary_engine
	This class outputs an empty plan for queries that use a secondary engine. More...
class	Query_result_null
	A query result handler that outputs nothing. More...
class	Find_thd_query_lock
	Callback function used by Sql_cmd_explain_other_thread::execute() to find thd based on the thread id. More...

Namespaces
namespace	anonymous_namespace{opt_explain.cc}

Typedefs
typedef qep_row::extra	extra

Functions
static bool	mysql_explain_query_expression (THD *explain_thd, const THD *query_thd, Query_expression *unit)
	Explain UNION or subqueries of the unit. More...
static bool	ExplainIterator (THD *ethd, const THD *query_thd, Query_expression *unit)
static bool	explain_ref_key (Explain_format *fmt, uint key_parts, store_key *key_copy[])
static bool	store_table_name (Table_ref *tr, Explain_format *fmt, std::function< bool(const char *name, size_t len)> func)
	Generates either usual table name or <derived::N>, and passes it to any given function for showing to the user. More...
static bool	explain_no_table (THD *explain_thd, const THD *query_thd, Query_block *query_block, const char *message, enum_parsing_context ctx)
	Send a message as an "extra" column value. More...
static bool	check_acl_for_explain (const Table_ref *table_list)
	Check that we are allowed to explain all views in list. More...
bool	explain_single_table_modification (THD *explain_thd, const THD *query_thd, const Modification_plan *plan, Query_block *select)
	EXPLAIN handling for single-table INSERT VALUES, UPDATE, and DELETE queries. More...
bool	explain_query_specification (THD *explain_thd, const THD *query_thd, Query_term *query_term, enum_parsing_context ctx)
	Explain query_block's join. More...
void	print_query_for_explain (const THD *query_thd, Query_expression *unit, String *str)
	This code which prints the extended description is not robust against malformed queries, so skip calling this function if we have an error or if explaining other thread (see Explain::can_print_clauses()). More...
bool	explain_query (THD *explain_thd, const THD *query_thd, Query_expression *unit)
	EXPLAIN handling for SELECT, INSERT/REPLACE SELECT, and multi-table UPDATE/DELETE queries. More...

Variables
const char *	join_type_str []
static const enum_query_type	cond_print_flags
static const char *	plan_not_ready []
	First string: for regular EXPLAIN; second: for EXPLAIN CONNECTION. More...

Detailed Description

"EXPLAIN <command>" implementation.

Typedef Documentation

extra

typedef qep_row::extra extra

Function Documentation

check_acl_for_explain()

static bool check_acl_for_explain ( const Table_ref *  table_list )

static

Check that we are allowed to explain all views in list.

Because this function is called only when we have a complete plan, we know that:

• views contained in merge-able views have been merged and brought up in the top list of tables, so we only need to scan this list
• table_list is not changing while we are reading it. If we don't have a complete plan, EXPLAIN output does not contain table names, so we don't need to check views.

Parameters

table_list

table to start with, usually lex->query_tables

Returns

true Caller can't EXPLAIN query due to lack of rights on a view in the query false Caller can EXPLAIN query

explain_no_table()

static bool explain_no_table ( THD *  explain_thd, const THD *  query_thd, Query_block *  query_block, const char *  message, enum_parsing_context  ctx  )

static

Send a message as an "extra" column value.

This function forms the 1st row of the QEP output with a simple text message. This is useful to explain such trivial cases as "No tables used" etc.

Note

Also this function explains the rest of QEP (subqueries or joined tables if any).

Parameters

explain_thd	thread handle for the connection doing explain
query_thd	thread handle for the connection being explained
query_block	query_block to explain
message	text message for the "extra" column.
ctx	current query context, CTX_JOIN in most cases.

Returns

false if success, true if error

explain_query()

bool explain_query	(	THD *	explain_thd,
		const THD *	query_thd,
		Query_expression *	unit
	)

EXPLAIN handling for SELECT, INSERT/REPLACE SELECT, and multi-table UPDATE/DELETE queries.

Send to the client a QEP data set for any DML statement that has a QEP represented completely by JOIN object(s).

This function uses a specific Query_result object for sending explain output to the client.

When explaining own query, the existing Query_result object (found in outermost Query_expression or Query_block) is used. However, if the Query_result is unsuitable for explanation (need_explain_interceptor() returns true), wrap the Query_result inside an Query_result_explain object.

When explaining other query, create a Query_result_send object and prepare it as if it was a regular SELECT query.

Note

see explain_single_table_modification() for single-table UPDATE/DELETE EXPLAIN handling.

explain_query() calls abort_result_set() itself in the case of failure (OOM etc.) since it may use an internally created Query_result object that has to be deleted before exiting the function.

Parameters

explain_thd	thread handle for the connection doing explain
query_thd	thread handle for the connection being explained
unit	query tree to explain

Returns

false if success, true if error

explain_query_specification()

bool explain_query_specification	(	THD *	explain_thd,
		const THD *	query_thd,
		Query_term *	query_term,
		enum_parsing_context	ctx
	)

Explain query_block's join.

Parameters

explain_thd	thread handle for the connection doing explain
query_thd	thread handle for the connection being explained
query_term	explain join attached to given term's query_block
ctx	current explain context

explain_ref_key()

static bool explain_ref_key ( Explain_format *  fmt, uint  key_parts, store_key *  key_copy[]  )

static

explain_single_table_modification()

bool explain_single_table_modification	(	THD *	explain_thd,
		const THD *	query_thd,
		const Modification_plan *	plan,
		Query_block *	select
	)

EXPLAIN handling for single-table INSERT VALUES, UPDATE, and DELETE queries.

Send to the client a QEP data set for single-table EXPLAIN INSERT VALUES/UPDATE/DELETE queries. As far as single-table INSERT VALUES/UPDATE/DELETE are implemented without the regular JOIN tree, we can't reuse explain_query_expression() directly, thus we deal with this single table in a special way and then call explain_query_expression() for subqueries (if any).

Parameters

explain_thd	thread handle for the connection doing explain
query_thd	thread handle for the connection being explained
plan	table modification plan
select	Query's select lex

Returns

false if success, true if error

Prepare the self-allocated result object

For queries with top-level JOIN the caller provides pre-allocated Query_result_send object. Then that JOIN object prepares the Query_result_send object calling result->prepare() in Query_block::prepare(), result->optimize() in JOIN::optimize() and result->start_execution() in JOIN::exec(). However without the presence of the top-level JOIN we have to prepare/initialize Query_result_send object manually.

ExplainIterator()

static bool ExplainIterator ( THD *  ethd, const THD *  query_thd, Query_expression *  unit  )

static

mysql_explain_query_expression()

bool mysql_explain_query_expression ( THD *  explain_thd, const THD *  query_thd, Query_expression *  unit  )

static

Explain UNION or subqueries of the unit.

If the unit is a UNION, explain it as a UNION. Otherwise explain nested subselects.

Parameters

explain_thd	thread handle for the connection doing explain
query_thd	thread handle for the connection being explained
unit	unit object, might not belong to ethd

Returns

false if success, true if error

print_query_for_explain()

void print_query_for_explain	(	const THD *	query_thd,
		Query_expression *	unit,
		String *	str
	)

This code which prints the extended description is not robust against malformed queries, so skip calling this function if we have an error or if explaining other thread (see Explain::can_print_clauses()).

For DML statements use QT_NO_DATA_EXPANSION to avoid over-simplification.

store_table_name()

static bool store_table_name ( Table_ref *  tr, Explain_format *  fmt, std::function< bool(const char *name, size_t len)>  func  )

static

Generates either usual table name or <derived::N>, and passes it to any given function for showing to the user.

Parameters

tr	Table reference
fmt	EXPLAIN's format
func	Function receiving the name

Returns

true if error.

Variable Documentation

cond_print_flags

const enum_query_type cond_print_flags

static

Initial value:

=
    enum_query_type(QT_ORDINARY | QT_SHOW_SELECT_NUMBER)

join_type_str

const char* join_type_str[]

Initial value:

= {
    "UNKNOWN", "system", "const",    "eq_ref",      "ref",        "ALL",
    "range",   "index",  "fulltext", "ref_or_null", "index_merge"}

plan_not_ready

const char* plan_not_ready[]

static

Initial value:

= {"Not optimized, outer query is empty",
                                       "Plan isn't ready yet"}

First string: for regular EXPLAIN; second: for EXPLAIN CONNECTION.