MySQL 8.4.3
Source Code Documentation
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EXPLAIN <command>. More...
#include "my_base.h"
#include "my_sqlcommand.h"
#include "my_thread_local.h"
#include "sql/opt_explain_format.h"
#include "sql/parse_tree_node_base.h"
#include "sql/query_result.h"
#include "sql/sql_cmd.h"
#include "sql/sql_opt_exec_shared.h"
#include "sys/types.h"
Go to the source code of this file.
Classes | |
class | Modification_plan |
Table modification plan for JOIN-less statements (update/delete) More... | |
class | Query_result_explain |
EXPLAIN functionality for Query_result_insert, Query_result_update and Query_result_delete. More... | |
class | Query_result_explain_into_var |
Wrapper class for writing EXPLAIN output to a user variable. More... | |
class | Sql_cmd_explain_other_thread |
Functions | |
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 (THD *explain_thd, const THD *query_thd, Query_expression *unit) |
EXPLAIN handling for SELECT, INSERT/REPLACE SELECT, and multi-table UPDATE/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... | |
Variables | |
const char * | join_type_str [] |
EXPLAIN <command>.
Single table UPDATE/DELETE commands are explained by the explain_single_table_modification() function.
A query expression (complete SELECT query possibly including subqueries and unions), INSERT...SELECT and multitable UPDATE/DELETE commands are explained like this:
(1) explain_query_expression()
Is the entry point. Forwards the job to explain_query_expression().
(2) explain_query_expression()
Is for a Query_expression, prepares, optimizes, explains one JOIN for each "top-level" Query_blocks of the unit (like: all SELECTs of a UNION; but not subqueries), and one or more JOIN for the post processing block Query_block of set operations (e.g. UNION), see query_term.h; each JOIN explain (JOIN::exec()) calls explain_query_specification()
(3) explain_query_specification()
Is for a single Query_block (post processing or not, see query_term.h). It needs a prepared and optimized JOIN, for which it builds the EXPLAIN rows. But it also launches the EXPLAIN process for "inner units" (==subqueries of this Query_block), by calling explain_query_expression() for each of them.
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.
explain_thd | thread handle for the connection doing explain |
query_thd | thread handle for the connection being explained |
unit | query tree to explain |
bool explain_query_specification | ( | THD * | explain_thd, |
const THD * | query_thd, | ||
Query_term * | query_term, | ||
enum_parsing_context | ctx | ||
) |
Explain query_block's join.
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 |
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).
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 |
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.
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.
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