/** @file @brief Table Elimination Module @defgroup Table_Elimination Table Elimination Module @{ */ #ifdef USE_PRAGMA_IMPLEMENTATION #pragma implementation // gcc: Class implementation #endif #include "mysql_priv.h" #include "my_bit.h" #include "sql_select.h" /* OVERVIEW This file contains table elimination module. The idea behind table elimination is as follows: suppose we have a left join SELECT * FROM t1 LEFT JOIN (t2 JOIN t3) ON t3.primary_key=t1.col AND t4.primary_key=t2.col such that * columns of the inner tables are not used anywhere ouside the outer join (not in WHERE, not in GROUP/ORDER BY clause, not in select list etc etc), * inner side of the outer join is guaranteed to produce at most one matching record combination for each record combination of outer tables. then the inner side of the outer join can be removed from the query, as it will always produce only one record combination (either real or null-complemented one) and we don't care about what that record combination is. MODULE INTERFACE The module has one entry point - eliminate_tables() function, which one needs to call (once) at some point before the join optimization. eliminate_tables() operates over the JOIN structures. Logically, it removes the right sides of outer join nests. Physically, it changes the following members: * Eliminated tables are marked as constant and moved to the front of the join order. * In addition to this, they are recorded in JOIN::eliminated_tables bitmap. * Items that became disused because they were in the ON expression of an eliminated outer join are notified by means of the Item tree walk which calls Item::mark_as_eliminated_processor for every item - At the moment the only Item that cares whether it was eliminated is Item_subselect with its Item_subselect::eliminated flag which is used by EXPLAIN code to check if the subquery should be shown in EXPLAIN. Table elimination is redone on every PS re-execution. TABLE ELIMINATION ALGORITHM As said above, we can remove inner side of an outer join if it is 1. not referred to from any other parts of the query 2. always produces one matching record combination. We check #1 by doing a recursive descent down the join->join_list while maintaining a union of used_tables() attribute of all expressions we've seen "elsewhere". When we encounter an outer join, we check if the bitmap of tables on its inner side has an intersection with tables that are used elsewhere. No intersection means that inner side of the outer join could potentially be eliminated. In order to check #2, one needs to prove that inner side of an outer join is functionally dependent on the outside. We prove dependency by proving functional dependency of intermediate objects: - Inner side of outer join is functionally dependent when each of its tables are functionally dependent. (We assume a table is functionally dependent when its dependencies allow to uniquely identify one table record, or no records). - Table is functionally dependent when it has got a unique key whose columns are functionally dependent. - A column is functionally dependent when we could locate an AND-part of a certain ON clause in form tblX.columnY= expr where expr is functionally-depdendent. Apparently the above rules can be applied recursively. Also, certain entities depend on multiple other entities. We model this by a bipartite graph which has two kinds of nodes: Value nodes: - Table column values (each is a value of tblX.columnY) - Table nodes (each node represents a table inside an eliminable join nest). each value is either bound (i.e. functionally dependent) or not. Module nodes: - Nodes representing tblX.colY=expr equalities. Equality node has = incoming edges from columns used in expr = outgoing edge to tblX.colY column. - Nodes representing unique keys. Unique key has = incoming edges from key component value nodes = outgoing edge to key's table node - Inner side of outer join node. Outer join node has = incoming edges from table value nodes = No outgoing edges. Once we reach it, we know we can eliminate the outer join. A module may depend on multiple values, and hence its primary attribute is the number of its depedencies that are not bound. The algorithm starts with equality nodes that don't have any incoming edges (their expressions are either constant or depend only on tables that are outside of any outer joins) and proceeds to traverse dependency->dependant edges until we've other traversed everything (TODO rephrase elaborate), or we've reached the point where all outer join modules have zero unsatisfied dependencies. */ class Value_dep; class Field_value; class Table_value; class Module_dep; class Equality_module; class Outer_join_module; class Key_module; class Table_elimination; /* A value. */ class Value_dep : public Sql_alloc { public: enum { VALUE_FIELD, VALUE_TABLE, } type; /* Type of the object */ Value_dep(): bound(FALSE), next(NULL) {} bool bound; Value_dep *next; }; /* A table field value. There is exactly only one such object for any tblX.fieldY - the field epends on its table and equalities - expressions that use the field are its dependencies */ class Field_value : public Value_dep { public: Field_value(Table_value *table_arg, Field *field_arg) : table(table_arg), field(field_arg) { type= Value_dep::VALUE_FIELD; } Table_value *table; /* Table this field is from */ Field *field; /* Field_deps that belong to one table form a linked list. list members are ordered by field_index */ Field_value *next_table_field; uint bitmap_offset; /* Offset of our part of the bitmap */ }; /* A table value. There is one Table_value object for every table that can potentially be eliminated. - table depends on any of its unique keys - has its fields and embedding outer join as dependency. */ class Table_value : public Value_dep { public: Table_value(TABLE *table_arg) : table(table_arg), fields(NULL), keys(NULL), outer_join_dep(NULL) { type= Value_dep::VALUE_TABLE; } TABLE *table; Field_value *fields; /* Ordered list of fields that belong to this table */ Key_module *keys; /* Ordered list of Unique keys in this table */ Outer_join_module *outer_join_dep; /* Innermost eliminable outer join we're in */ }; /* A 'module' */ class Module_dep : public Sql_alloc { public: enum { MODULE_EXPRESSION, MODULE_MULTI_EQUALITY, MODULE_UNIQUE_KEY, MODULE_OUTER_JOIN } type; /* Type of the object */ /* Used to make a linked list of elements that became bound and thus can make elements that depend on them bound, too. */ Module_dep *next; uint unknown_args; Module_dep() : next(NULL), unknown_args(0) {} }; /* A "tbl.column= expr" equality dependency. tbl.column depends on fields used in expr. */ class Equality_module : public Module_dep { public: Field_value *field; Item *expression; /* Used during condition analysis only, similar to KEYUSE::level */ uint level; }; /* A Unique key. - Unique key depends on all of its components - Key's table is its dependency */ class Key_module: public Module_dep { public: Key_module(Table_value *table_arg, uint keyno_arg, uint n_parts_arg) : table(table_arg), keyno(keyno_arg), next_table_key(NULL) { type= Module_dep::MODULE_UNIQUE_KEY; unknown_args= n_parts_arg; } Table_value *table; /* Table this key is from */ uint keyno; /* Unique keys form a linked list, ordered by keyno */ Key_module *next_table_key; }; /* An outer join nest that is subject to elimination - it depends on all tables inside it - has its parent outer join as dependency */ class Outer_join_module: public Module_dep { public: Outer_join_module(TABLE_LIST *table_list_arg, uint n_children) : table_list(table_list_arg), parent(NULL) { type= Module_dep::MODULE_OUTER_JOIN; unknown_args= n_children; } /* Outer join we're representing. This can be a join nest or one table that is outer join'ed. */ TABLE_LIST *table_list; /* Parent eliminable outer join, if any */ Outer_join_module *parent; }; /* Table elimination context */ class Table_elimination { public: Table_elimination(JOIN *join_arg) : join(join_arg), n_outer_joins(0) { bzero(table_deps, sizeof(table_deps)); } JOIN *join; /* Array of equality dependencies */ Equality_module *equality_deps; uint n_equality_deps; /* Number of elements in the array */ /* tablenr -> Table_value* mapping. */ Table_value *table_deps[MAX_KEY]; /* Outer joins that are candidates for elimination */ List<Outer_join_module> oj_deps; uint n_outer_joins; /* Bitmap of how expressions depend on bits */ MY_BITMAP expr_deps; }; static bool build_eq_deps_for_cond(Table_elimination *te, Equality_module **fdeps, uint *and_level, Item *cond, table_map usable_tables); static bool add_eq_dep(Table_elimination *te, Equality_module **eq_dep, uint and_level, Item_func *cond, Item *left, Item *right, table_map usable_tables); static Equality_module *merge_func_deps(Equality_module *start, Equality_module *new_fields, Equality_module *end, uint and_level); static Table_value *get_table_value(Table_elimination *te, TABLE *table); static Field_value *get_field_value(Table_elimination *te, Field *field); static void run_elimination_wave(Table_elimination *te, Module_dep *bound_modules); void eliminate_tables(JOIN *join); static void mark_as_eliminated(JOIN *join, TABLE_LIST *tbl); #ifndef DBUG_OFF static void dbug_print_deps(Table_elimination *te); #endif /*******************************************************************************************/ /* Produce Eq_dep elements for given condition. SYNOPSIS build_eq_deps_for_cond() te Table elimination context fdeps INOUT Put produced equality conditions here and_level INOUT AND-level (like in add_key_fields) cond Condition to process usable_tables Tables which fields we're interested in. That is, Equality_dep represent "tbl.col=expr" and we'll produce them only if tbl is in usable_tables. DESCRIPTION This function is modeled after add_key_fields() */ static bool build_eq_deps_for_cond(Table_elimination *te, Equality_module **fdeps, uint *and_level, Item *cond, table_map usable_tables) { if (cond->type() == Item_func::COND_ITEM) { List_iterator_fast<Item> li(*((Item_cond*) cond)->argument_list()); Equality_module *org_key_fields= *fdeps; /* AND/OR */ if (((Item_cond*) cond)->functype() == Item_func::COND_AND_FUNC) { Item *item; while ((item=li++)) { if (build_eq_deps_for_cond(te, fdeps, and_level, item, usable_tables)) return TRUE; } /* TODO: inject here a "if we have {t.col=const AND t.col=smth_else}, then remove the second part" logic. */ for (; org_key_fields != *fdeps ; org_key_fields++) org_key_fields->level= *and_level; } else { (*and_level)++; if (build_eq_deps_for_cond(te, fdeps, and_level, li++, usable_tables)) return TRUE; Item *item; while ((item=li++)) { Equality_module *start_key_fields= *fdeps; (*and_level)++; if (build_eq_deps_for_cond(te, fdeps, and_level, item, usable_tables)) return TRUE; *fdeps= merge_func_deps(org_key_fields, start_key_fields, *fdeps, ++(*and_level)); } } return FALSE; } if (cond->type() != Item::FUNC_ITEM) return FALSE; Item_func *cond_func= (Item_func*) cond; Item **args= cond_func->arguments(); switch (cond_func->functype()) { case Item_func::IN_FUNC: { if (cond_func->argument_count() == 2) { if (add_eq_dep(te, fdeps, *and_level, cond_func, args[0], args[1], usable_tables) || add_eq_dep(te, fdeps, *and_level, cond_func, args[1], args[0], usable_tables)) return TRUE; } } case Item_func::BETWEEN: { Item *fld; if (!((Item_func_between*)cond)->negated && (fld= args[0]->real_item())->type() == Item::FIELD_ITEM && args[1]->eq(args[2], ((Item_field*)fld)->field->binary())) { if (add_eq_dep(te, fdeps, *and_level, cond_func, args[0], args[1], usable_tables) || add_eq_dep(te, fdeps, *and_level, cond_func, args[1], args[0], usable_tables)) return TRUE; } break; } case Item_func::EQ_FUNC: case Item_func::EQUAL_FUNC: { add_eq_dep(te, fdeps, *and_level, cond_func, args[0], args[1], usable_tables); add_eq_dep(te, fdeps, *and_level, cond_func, args[1], args[0], usable_tables); break; } case Item_func::ISNULL_FUNC: { Item *tmp=new Item_null; if (!tmp || add_eq_dep(te, fdeps, *and_level, cond_func, args[0], args[1], usable_tables)) return TRUE; break; } case Item_func::MULT_EQUAL_FUNC: { Item_equal *item_equal= (Item_equal *) cond; Item *const_item= item_equal->get_const(); Item_equal_iterator it(*item_equal); Item_field *item; if (const_item) { /* For each field field1 from item_equal consider the equality field1=const_item as a condition allowing an index access of the table with field1 by the keys value of field1. */ while ((item= it++)) { if (add_eq_dep(te, fdeps, *and_level, cond_func, item, const_item, usable_tables)) return TRUE; } } else { /* Consider all pairs of different fields included into item_equal. For each of them (field1, field1) consider the equality field1=field2 as a condition allowing an index access of the table with field1 by the keys value of field2. */ Item_equal_iterator fi(*item_equal); while ((item= fi++)) { Field *field= item->field; Item_field *item2; while ((item2= it++)) { if (!field->eq(item2->field)) { if (add_eq_dep(te, fdeps, *and_level, cond_func, item, item2, usable_tables)) return TRUE; } } it.rewind(); } } break; } default: break; } return FALSE; } /* Perform an OR operation on two (adjacent) Equality_module arrays. SYNOPSIS merge_func_deps() start Start of left OR-part new_fields Start of right OR-part end End of right OR-part and_level AND-level. DESCRIPTION This function is invoked for two adjacent arrays of Equality_module elements: $LEFT_PART $RIGHT_PART +-----------------------+-----------------------+ start new_fields end The goal is to produce an array which would correspnd to the combined $LEFT_PART OR $RIGHT_PART condition. This is achieved as follows: First, we apply distrubutive law: (fdep_A_1 AND fdep_A_2 AND ...) OR (fdep_B_1 AND fdep_B_2 AND ...) = = AND_ij (fdep_A_[i] OR fdep_B_[j]) Then we walk over the obtained "fdep_A_[i] OR fdep_B_[j]" pairs, and - Discard those that that have left and right part referring to different columns. We can't infer anything useful from "col1=expr1 OR col2=expr2". - When left and right parts refer to the same column, we check if they are essentially the same. = If they are the same, we keep one copy "t.col=expr OR t.col=expr" -> "t.col=expr = if they are different , then we discard both "t.col=expr1 OR t.col=expr2" -> (nothing useful) (no per-table or for-index FUNC_DEPS exist yet at this phase). See also merge_key_fields(). RETURN End of the result array */ static Equality_module *merge_func_deps(Equality_module *start, Equality_module *new_fields, Equality_module *end, uint and_level) { if (start == new_fields) return start; // Impossible or if (new_fields == end) return start; // No new fields, skip all Equality_module *first_free=new_fields; for (; new_fields != end ; new_fields++) { for (Equality_module *old=start ; old != first_free ; old++) { /* TODO: does it make sense to attempt to merging multiple-equalities? A: YES. (a=b=c) OR (a=b=d) produce "a=b". QQ: What to use for merging? Trivial N*M algorithm or pre-sort and then merge ordered sequences? */ if (old->field == new_fields->field) { if (!new_fields->expression->const_item()) { /* If the value matches, we can use the key reference. If not, we keep it until we have examined all new values */ if (old->expression->eq(new_fields->expression, old->field->field->binary())) { old->level= and_level; } } else if (old->expression->eq_by_collation(new_fields->expression, old->field->field->binary(), old->field->field->charset())) { old->level= and_level; } else { /* The expressions are different. */ if (old == --first_free) // If last item break; *old= *first_free; // Remove old value old--; // Retry this value } } } } /* Ok, the results are within the [start, first_free) range, and the useful elements have level==and_level. Now, lets remove all unusable elements: */ for (Equality_module *old=start ; old != first_free ;) { if (old->level != and_level) { // Not used in all levels if (old == --first_free) break; *old= *first_free; // Remove old value continue; } old++; } return first_free; } /* Add an Equality_module element for a given predicate, if applicable DESCRIPTION This function is modeled after add_key_field(). */ static bool add_eq_dep(Table_elimination *te, Equality_module **eq_dep, uint and_level, Item_func *cond, Item *left, Item *right, table_map usable_tables) { if ((left->used_tables() & usable_tables) && !(right->used_tables() & RAND_TABLE_BIT) && left->real_item()->type() == Item::FIELD_ITEM) { Field *field= ((Item_field*)left->real_item())->field; if (field->result_type() == STRING_RESULT) { if (right->result_type() != STRING_RESULT) { if (field->cmp_type() != right->result_type()) return FALSE; } else { /* We can't use indexes if the effective collation of the operation differ from the field collation. */ if (field->cmp_type() == STRING_RESULT && ((Field_str*)field)->charset() != cond->compare_collation()) return FALSE; } } (*eq_dep)->type= Module_dep::MODULE_EXPRESSION; //psergey-todo; if (!((*eq_dep)->field= get_field_value(te, field))) return TRUE; (*eq_dep)->expression= right; (*eq_dep)->level= and_level; (*eq_dep)++; } return FALSE; } /* Get a Table_value object for the given table, creating it if necessary. */ static Table_value *get_table_value(Table_elimination *te, TABLE *table) { Table_value *tbl_dep; if (!(tbl_dep= new Table_value(table))) return NULL; Key_module **key_list= &(tbl_dep->keys); /* Add dependencies for unique keys */ for (uint i=0; i < table->s->keys; i++) { KEY *key= table->key_info + i; if ((key->flags & (HA_NOSAME | HA_END_SPACE_KEY)) == HA_NOSAME) { Key_module *key_dep= new Key_module(tbl_dep, i, key->key_parts); *key_list= key_dep; key_list= &(key_dep->next_table_key); } } return te->table_deps[table->tablenr]= tbl_dep; } /* Get a Field_value object for the given field, creating it if necessary */ static Field_value *get_field_value(Table_elimination *te, Field *field) { TABLE *table= field->table; Table_value *tbl_dep; /* First, get the table*/ if (!(tbl_dep= te->table_deps[table->tablenr])) { if (!(tbl_dep= get_table_value(te, table))) return NULL; } /* Try finding the field in field list */ Field_value **pfield= &(tbl_dep->fields); while (*pfield && (*pfield)->field->field_index < field->field_index) { pfield= &((*pfield)->next_table_field); } if (*pfield && (*pfield)->field->field_index == field->field_index) return *pfield; /* Create the field and insert it in the list */ Field_value *new_field= new Field_value(tbl_dep, field); new_field->next_table_field= *pfield; *pfield= new_field; return new_field; } /* Create an Outer_join_module object for the given outer join DESCRIPTION Outer_join_module objects for children (or further descendants) are always created before the parents. */ static Outer_join_module *get_outer_join_dep(Table_elimination *te, TABLE_LIST *outer_join, table_map deps_map) { Outer_join_module *oj_dep; if (!(oj_dep= new Outer_join_module(outer_join, my_count_bits(deps_map)))) return NULL; te->n_outer_joins++; /* Collect a bitmap fo tables that we depend on, and also set parent pointer for descendant outer join elements. */ Table_map_iterator it(deps_map); int idx; while ((idx= it.next_bit()) != Table_map_iterator::BITMAP_END) { Table_value *table_dep; if (!(table_dep= te->table_deps[idx])) { /* We get here only when ON expression had no references to inner tables and Table_map objects weren't created for them. This is a rare/ unimportant case so it's ok to do not too efficient searches. */ TABLE *table= NULL; for (TABLE_LIST *tlist= te->join->select_lex->leaf_tables; tlist; tlist=tlist->next_leaf) { if (tlist->table->tablenr == (uint)idx) { table=tlist->table; break; } } DBUG_ASSERT(table); if (!(table_dep= get_table_value(te, table))) return NULL; } /* Walk from the table up to its embedding outer joins. The goal is to find the least embedded outer join nest and set its parent pointer to point to the newly created Outer_join_module. to set the pointer of its near */ if (!table_dep->outer_join_dep) table_dep->outer_join_dep= oj_dep; else { Outer_join_module *oj= table_dep->outer_join_dep; while (oj->parent) oj= oj->parent; if (oj != oj_dep) oj->parent=oj_dep; } } return oj_dep; } /* Build functional dependency graph for elements of given join list SYNOPSIS collect_funcdeps_for_join_list() te Table elimination context. join_list Join list to work on build_eq_deps TRUE <=> build Equality_module elements for all members of the join list, even if they cannot be individually eliminated tables_used_elsewhere Bitmap of tables that are referred to from somewhere outside of this join list (e.g. select list, HAVING, ON expressions of parent joins, etc). eliminable_tables INOUT Tables that can potentially be eliminated (needed so we know for which tables to build dependencies for) eq_dep INOUT End of array of equality dependencies. DESCRIPTION . */ static bool collect_funcdeps_for_join_list(Table_elimination *te, List<TABLE_LIST> *join_list, bool build_eq_deps, table_map tables_used_elsewhere, table_map *eliminable_tables, Equality_module **eq_dep) { TABLE_LIST *tbl; List_iterator<TABLE_LIST> it(*join_list); table_map tables_used_on_left= 0; while ((tbl= it++)) { if (tbl->on_expr) { table_map outside_used_tables= tables_used_elsewhere | tables_used_on_left; bool eliminable; table_map cur_map; if (tbl->nested_join) { /* This is "... LEFT JOIN (join_nest) ON cond" */ cur_map= tbl->nested_join->used_tables; eliminable= !(cur_map & outside_used_tables); if (eliminable) *eliminable_tables |= cur_map; if (collect_funcdeps_for_join_list(te, &tbl->nested_join->join_list, eliminable || build_eq_deps, outside_used_tables, eliminable_tables, eq_dep)) return TRUE; } else { /* This is "... LEFT JOIN tbl ON cond" */ cur_map= tbl->table->map; eliminable= !(tbl->table->map & outside_used_tables); *eliminable_tables |= cur_map; } if (eliminable || build_eq_deps) { // build comp_cond from ON expression uint and_level=0; build_eq_deps_for_cond(te, eq_dep, &and_level, tbl->on_expr, *eliminable_tables); } if (eliminable && !get_outer_join_dep(te, tbl, cur_map)) return TRUE; tables_used_on_left |= tbl->on_expr->used_tables(); } } return FALSE; } /* This is used to analyze expressions in "tbl.col=expr" dependencies so that we can figure out which fields the expression depends on. */ class Field_dependency_setter : public Field_enumerator { public: Field_dependency_setter(Table_elimination *te_arg): te(te_arg) {} void see_field(Field *field) { Table_value *tbl_dep; if ((tbl_dep= te->table_deps[field->table->tablenr])) { for (Field_value *field_dep= tbl_dep->fields; field_dep; field_dep= field_dep->next_table_field) { if (field->field_index == field_dep->field->field_index) { uint offs= field_dep->bitmap_offset + expr_offset; if (!bitmap_is_set(&te->expr_deps, offs)) te->equality_deps[expr_offset].unknown_args++; bitmap_set_bit(&te->expr_deps, offs); return; } } /* We got here if didn't find this field. It's not a part of a unique key, and/or there is no field=expr element for it. Bump the dependency anyway, this will signal that this dependency cannot be satisfied. */ te->equality_deps[expr_offset].unknown_args++; } } Table_elimination *te; /* Offset of the expression we're processing in the dependency bitmap */ uint expr_offset; }; /* Setup equality dependencies SYNOPSIS setup_equality_deps() te Table elimination context bound_deps_list OUT Start of linked list of elements that were found to be bound (caller will use this to see if that allows to declare further elements bound) */ static bool setup_equality_deps(Table_elimination *te, Module_dep **bound_deps_list) { DBUG_ENTER("setup_equality_deps"); /* Count Field_value objects and assign each of them a unique bitmap_offset. */ uint offset= 0; for (Table_value **tbl_dep=te->table_deps; tbl_dep < te->table_deps + MAX_TABLES; tbl_dep++) { if (*tbl_dep) { for (Field_value *field_dep= (*tbl_dep)->fields; field_dep; field_dep= field_dep->next_table_field) { field_dep->bitmap_offset= offset; offset += te->n_equality_deps; } } } void *buf; if (!(buf= current_thd->alloc(bitmap_buffer_size(offset))) || bitmap_init(&te->expr_deps, (my_bitmap_map*)buf, offset, FALSE)) { DBUG_RETURN(TRUE); } bitmap_clear_all(&te->expr_deps); /* Analyze all "field=expr" dependencies, and have te->expr_deps encode dependencies of expressions from fields. Also collect a linked list of equalities that are bound. */ Module_dep *bound_dep= NULL; Field_dependency_setter deps_setter(te); for (Equality_module *eq_dep= te->equality_deps; eq_dep < te->equality_deps + te->n_equality_deps; eq_dep++) { deps_setter.expr_offset= eq_dep - te->equality_deps; eq_dep->unknown_args= 0; eq_dep->expression->walk(&Item::check_column_usage_processor, FALSE, (uchar*)&deps_setter); if (!eq_dep->unknown_args) { eq_dep->next= bound_dep; bound_dep= eq_dep; } } *bound_deps_list= bound_dep; DBUG_EXECUTE("test", dbug_print_deps(te); ); DBUG_RETURN(FALSE); } /* Perform table elimination SYNOPSIS eliminate_tables() join Join to work on const_tbl_count INOUT Number of constant tables (this includes eliminated tables) const_tables INOUT Bitmap of constant tables DESCRIPTION This function is the entry point for table elimination. The idea behind table elimination is that if we have an outer join: SELECT * FROM t1 LEFT JOIN (t2 JOIN t3) ON t3.primary_key=t1.col AND t4.primary_key=t2.col such that 1. columns of the inner tables are not used anywhere ouside the outer join (not in WHERE, not in GROUP/ORDER BY clause, not in select list etc etc), and 2. inner side of the outer join is guaranteed to produce at most one record combination for each record combination of outer tables. then the inner side of the outer join can be removed from the query. This is because it will always produce one matching record (either a real match or a NULL-complemented record combination), and since there are no references to columns of the inner tables anywhere, it doesn't matter which record combination it was. This function primary handles checking #1. It collects a bitmap of tables that are not used in select list/GROUP BY/ORDER BY/HAVING/etc and thus can possibly be eliminated. SIDE EFFECTS See the OVERVIEW section at the top of this file. */ void eliminate_tables(JOIN *join) { THD* thd= join->thd; Item *item; table_map used_tables; DBUG_ENTER("eliminate_tables"); DBUG_ASSERT(join->eliminated_tables == 0); /* If there are no outer joins, we have nothing to eliminate: */ if (!join->outer_join) DBUG_VOID_RETURN; /* Find the tables that are referred to from WHERE/HAVING */ used_tables= (join->conds? join->conds->used_tables() : 0) | (join->having? join->having->used_tables() : 0); /* Add tables referred to from the select list */ List_iterator<Item> it(join->fields_list); while ((item= it++)) used_tables |= item->used_tables(); /* Add tables referred to from ORDER BY and GROUP BY lists */ ORDER *all_lists[]= { join->order, join->group_list}; for (int i=0; i < 2; i++) { for (ORDER *cur_list= all_lists[i]; cur_list; cur_list= cur_list->next) used_tables |= (*(cur_list->item))->used_tables(); } if (join->select_lex == &thd->lex->select_lex) { /* Multi-table UPDATE and DELETE: don't eliminate the tables we modify: */ used_tables |= thd->table_map_for_update; /* Multi-table UPDATE: don't eliminate tables referred from SET statement */ if (thd->lex->sql_command == SQLCOM_UPDATE_MULTI) { List_iterator<Item> it2(thd->lex->value_list); while ((item= it2++)) used_tables |= item->used_tables(); } } table_map all_tables= join->all_tables_map(); if (all_tables & ~used_tables) { /* There are some tables that we probably could eliminate. Try it. */ Table_elimination te(join); uint m= max(thd->lex->current_select->max_equal_elems,1); uint max_elems= ((thd->lex->current_select->cond_count+1)*2 + thd->lex->current_select->between_count)*m + 1 + 10; if (!(te.equality_deps= new Equality_module[max_elems])) DBUG_VOID_RETURN; Equality_module *eq_deps_end= te.equality_deps; table_map eliminable_tables= 0; if (collect_funcdeps_for_join_list(&te, join->join_list, FALSE, used_tables, &eliminable_tables, &eq_deps_end)) DBUG_VOID_RETURN; te.n_equality_deps= eq_deps_end - te.equality_deps; Module_dep *bound_modules; //Value_dep *bound_values; if (setup_equality_deps(&te, &bound_modules)) DBUG_VOID_RETURN; run_elimination_wave(&te, bound_modules); } DBUG_VOID_RETURN; } static void signal_from_field_to_exprs(Table_elimination* te, Field_value *field_dep, Module_dep **bound_modules) { for (uint i=0; i < te->n_equality_deps; i++) { if (bitmap_is_set(&te->expr_deps, field_dep->bitmap_offset + i) && te->equality_deps[i].unknown_args && !--te->equality_deps[i].unknown_args) { /* Mark as bound and add to the list */ Equality_module* eq_dep= &te->equality_deps[i]; eq_dep->next= *bound_modules; *bound_modules= eq_dep; } } } static void run_elimination_wave(Table_elimination *te, Module_dep *bound_modules) { Value_dep *bound_values= NULL; /* Run the wave. All Func_dep-derived objects are divided into three classes: - Those that have bound=FALSE - Those that have bound=TRUE - Those that have bound=TRUE and are in the list.. */ while (bound_modules) { for (;bound_modules; bound_modules= bound_modules->next) { switch (bound_modules->type) { case Module_dep::MODULE_EXPRESSION: { /* It's a field=expr and we got to know the expr, so we know the field */ Equality_module *eq_dep= (Equality_module*)bound_modules; if (!eq_dep->field->bound) { /* Mark as bound and add to the list */ eq_dep->field->bound= TRUE; eq_dep->field->next= bound_values; bound_values= eq_dep->field; } break; } case Module_dep::MODULE_UNIQUE_KEY: { /* Unique key is known means the table is known */ Table_value *table_dep=((Key_module*)bound_modules)->table; if (!table_dep->bound) { /* Mark as bound and add to the list */ table_dep->bound= TRUE; table_dep->next= bound_values; bound_values= table_dep; } break; } case Module_dep::MODULE_OUTER_JOIN: { Outer_join_module *outer_join_dep= (Outer_join_module*)bound_modules; mark_as_eliminated(te->join, outer_join_dep->table_list); if (!--te->n_outer_joins) { DBUG_PRINT("info", ("Table elimination eliminated everything" " it theoretically could")); return; } break; } case Module_dep::MODULE_MULTI_EQUALITY: default: DBUG_ASSERT(0); } } for (;bound_values; bound_values=bound_values->next) { switch (bound_values->type) { case Value_dep::VALUE_FIELD: { /* Field became known. Check out - unique keys we belong to - expressions that depend on us. */ Field_value *field_dep= (Field_value*)bound_values; for (Key_module *key_dep= field_dep->table->keys; key_dep; key_dep= key_dep->next_table_key) { DBUG_PRINT("info", ("key %s.%s is now bound", key_dep->table->table->alias, key_dep->table->table->key_info[key_dep->keyno].name)); if (field_dep->field->part_of_key.is_set(key_dep->keyno) && key_dep->unknown_args && !--key_dep->unknown_args) { /* Mark as bound and add to the list */ key_dep->next= bound_modules; bound_modules= key_dep; } } signal_from_field_to_exprs(te, field_dep, &bound_modules); break; } case Value_dep::VALUE_TABLE: { Table_value *table_dep=(Table_value*)bound_values; DBUG_PRINT("info", ("table %s is now bound", table_dep->table->alias)); /* Table is known means - all its fields are known - one more element in outer join nest is known */ for (Field_value *field_dep= table_dep->fields; field_dep; field_dep= field_dep->next_table_field) { if (!field_dep->bound) { /* Mark as bound and add to the list */ field_dep->bound= TRUE; signal_from_field_to_exprs(te, field_dep, &bound_modules); } } for (Outer_join_module *outer_join_dep= table_dep->outer_join_dep; outer_join_dep; outer_join_dep= outer_join_dep->parent) { if (outer_join_dep->unknown_args && !--outer_join_dep->unknown_args) { /* Mark as bound and add to the list */ outer_join_dep->next= bound_modules; bound_modules= outer_join_dep; } } break; } default: DBUG_ASSERT(0); } } } } /* Mark one table or the whole join nest as eliminated. */ static void mark_as_eliminated(JOIN *join, TABLE_LIST *tbl) { TABLE *table; /* NOTE: there are TABLE_LIST object that have tbl->table!= NULL && tbl->nested_join!=NULL and tbl->table == tbl->nested_join->join_list->element(..)->table */ if (tbl->nested_join) { TABLE_LIST *child; List_iterator<TABLE_LIST> it(tbl->nested_join->join_list); while ((child= it++)) mark_as_eliminated(join, child); } else if ((table= tbl->table)) { JOIN_TAB *tab= tbl->table->reginfo.join_tab; if (!(join->const_table_map & tab->table->map)) { DBUG_PRINT("info", ("Eliminated table %s", table->alias)); tab->type= JT_CONST; join->eliminated_tables |= table->map; join->const_table_map|= table->map; set_position(join, join->const_tables++, tab, (KEYUSE*)0); } } if (tbl->on_expr) tbl->on_expr->walk(&Item::mark_as_eliminated_processor, FALSE, NULL); } #ifndef DBUG_OFF static void dbug_print_deps(Table_elimination *te) { DBUG_ENTER("dbug_print_deps"); DBUG_LOCK_FILE; fprintf(DBUG_FILE,"deps {\n"); /* Start with printing equalities */ for (Equality_module *eq_dep= te->equality_deps; eq_dep != te->equality_deps + te->n_equality_deps; eq_dep++) { char buf[128]; String str(buf, sizeof(buf), &my_charset_bin); str.length(0); eq_dep->expression->print(&str, QT_ORDINARY); fprintf(DBUG_FILE, " equality%d: %s -> %s.%s\n", eq_dep - te->equality_deps, str.c_ptr(), eq_dep->field->table->table->alias, eq_dep->field->field->field_name); } fprintf(DBUG_FILE,"\n"); /* Then tables and their fields */ for (uint i=0; i < MAX_TABLES; i++) { Table_value *table_dep; if ((table_dep= te->table_deps[i])) { /* Print table */ fprintf(DBUG_FILE, " table %s\n", table_dep->table->alias); /* Print fields */ for (Field_value *field_dep= table_dep->fields; field_dep; field_dep= field_dep->next_table_field) { fprintf(DBUG_FILE, " field %s.%s ->", table_dep->table->alias, field_dep->field->field_name); uint ofs= field_dep->bitmap_offset; for (uint bit= ofs; bit < ofs + te->n_equality_deps; bit++) { if (bitmap_is_set(&te->expr_deps, bit)) fprintf(DBUG_FILE, " equality%d ", bit - ofs); } fprintf(DBUG_FILE, "\n"); } } } fprintf(DBUG_FILE,"\n}\n"); DBUG_UNLOCK_FILE; DBUG_VOID_RETURN; } #endif /** @} (end of group Table_Elimination) */