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Mark constant 'null_tables' with table->const_table=1

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This was done to make thing consistent. It gives the additional benefit
that EXPLAIN EXTENDED now treat null_tables like constant's and replaces
columns with NULL, in a similar way that it replaces columns with constants
for constant tables.

- Null tables are tables where all columns are always NULL. The most common
  NULL TABLE is a table used in a LEFT_JOIN that is never true.
- All result changes comes from replacing columns with NULL for null_tables.
- "Impossible where" is now also shows constants for const columns.
- Removed duplicated s->type= JT_CONST
- Reset found_const_table_map when JOIN is created (safety fix)
This commit is contained in:
Monty
2017-11-30 17:01:23 +02:00
parent ddac2d7a1e
commit c65911ac46
10 changed files with 15 additions and 11 deletions
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5
sql/sql_select.cc
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@ -4198,6 +4198,7 @@ make_join_statistics(JOIN *join, List<TABLE_LIST> &tables_list,
keyuse->val->is_null() && keyuse->null_rejecting)
{
s->type= JT_CONST;
s->table->const_table= 1;
mark_as_null_row(table);
found_const_table_map|= table->map;
join->const_table_map|= table->map;
@ -4303,6 +4304,7 @@ make_join_statistics(JOIN *join, List<TABLE_LIST> &tables_list,
s->type= JT_CONST;
join->const_table_map|=table->map;
set_position(join,const_count++,s,start_keyuse);
/* create_ref_for_key will set s->table->const_table */
if (create_ref_for_key(join, s, start_keyuse, FALSE,
found_const_table_map))
goto error;
@ -4508,12 +4510,12 @@ make_join_statistics(JOIN *join, List<TABLE_LIST> &tables_list,
join->const_table_map|= s->table->map;
set_position(join,const_count++,s,(KEYUSE*) 0);
s->type= JT_CONST;
s->table->const_table= 1;
if (*s->on_expr_ref)
{
/* Generate empty row */
s->info= ET_IMPOSSIBLE_ON_CONDITION;
found_const_table_map|= s->table->map;
s->type= JT_CONST;
mark_as_null_row(s->table); // All fields are NULL
}
}
@ -19533,6 +19535,7 @@ join_read_system(JOIN_TAB *tab)
{
if (error != HA_ERR_END_OF_FILE)
return report_error(table, error);
table->const_table= 1;
mark_as_null_row(tab->table);
empty_record(table); // Make empty record
return -1;