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montywi
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MDEV-6080: Allowing storage engine to shortcut group by queries
This task is to allow storage engines that can execute GROUP BY or summary queries efficiently to intercept a full query or sub query from MariaDB and deliver the result either to the client or to a temporary table for further processing. - Added code in sql_select.cc to intercept GROUP BY queries. Creation of group_by_handler is done after all optimizations to allow storage engine to benefit of an optimized WHERE clause and suggested indexes to use. - Added group by handler to sequence engine and a group_by test suite as a way to test the new interface. - Intercept EXPLAIN with a message "Storage engine handles GROUP BY" libmysqld/CMakeLists.txt: Added new group_by_handler files sql/CMakeLists.txt: Added new group_by_handler files sql/group_by_handler.cc: Implementation of group_by_handler functions sql/group_by_handler.h: Definition of group_by_handler class sql/handler.h: Added handlerton function to create a group_by_handler, if the storage engine can intercept the query. sql/item_cmpfunc.cc: Allow one to evaluate item_equal any time. sql/sql_select.cc: Added code to intercept GROUP BY queries - If all tables are from the same storage engine and the query is using sum functions, call create_group_by() to check if the storage engine can intercept the query. - If yes: - create a temporary table to hold a GROUP_BY row or result - In do_select() intercept normal query execution by instead calling the group_by_handler to get the result - Intercept EXPLAIN sql/sql_select.h: Added handling of group_by_handler Added caching of the original join tab (needed for cleanup after group_by handler) storage/sequence/mysql-test/sequence/group_by.result: Test group_by_handler interface storage/sequence/mysql-test/sequence/group_by.test: Test group_by_handler interface storage/sequence/sequence.cc: Added simple group_by_engine for handling COUNT(*) and SUM(primary_key). This was done as a test of the group_by_handler interface
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‎libmysqld/CMakeLists.txt‎

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@@ -75,6 +75,7 @@ SET(SQL_EMBEDDED_SOURCES emb_qcache.cc libmysqld.c lib_sql.cc
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../sql/debug_sync.cc ../sql/opt_table_elimination.cc
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../sql/sql_prepare.cc ../sql/sql_rename.cc ../sql/sql_repl.cc
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../sql/sql_select.cc ../sql/sql_servers.cc
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../sql/group_by_handler.cc
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../sql/sql_show.cc ../sql/sql_state.c
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../sql/sql_statistics.cc ../sql/sql_string.cc
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../sql/sql_tablespace.cc ../sql/sql_table.cc ../sql/sql_test.cc

‎sql/CMakeLists.txt‎

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@@ -108,6 +108,7 @@ SET (SQL_SOURCE
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sql_partition.cc sql_plugin.cc sql_prepare.cc sql_rename.cc
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debug_sync.cc debug_sync.h
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sql_repl.cc sql_select.cc sql_show.cc sql_state.c
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group_by_handler.cc
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sql_statistics.cc sql_string.cc
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sql_table.cc sql_test.cc sql_trigger.cc sql_udf.cc sql_union.cc
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sql_update.cc sql_view.cc strfunc.cc table.cc thr_malloc.cc

‎sql/group_by_handler.cc‎

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@@ -0,0 +1,144 @@
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/*
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Copyright (c) 2014, SkySQL Ab & MariaDB Foundation
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; version 2 of the License.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
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/*
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This file implements the group_by_handler code. This interface
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can be used by storage handlers that can intercept summary or GROUP
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BY queries from MariaDB and itself return the result to the user or
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upper level.
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*/
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#ifdef USE_PRAGMA_IMPLEMENTATION
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#pragma implementation // gcc: Class implementation
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#endif
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#include "sql_priv.h"
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#include "sql_select.h"
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/*
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Same return values as do_select();
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@retval
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0 if ok
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@retval
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1 if error is sent
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@retval
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-1 if error should be sent
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*/
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int group_by_handler::execute(JOIN *join)
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{
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int err;
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ha_rows max_limit;
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ha_rows *reset_limit= 0;
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Item **reset_item= 0;
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DBUG_ENTER("group_by_handler");
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if ((err= init_scan()))
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goto error;
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if (store_data_in_temp_table)
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{
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max_limit= join->tmp_table_param.end_write_records;
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reset_limit= &join->unit->select_limit_cnt;
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}
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else
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{
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max_limit= join->unit->select_limit_cnt;
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if (join->unit->fake_select_lex)
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reset_item= &join->unit->fake_select_lex->select_limit;
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}
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while (!(err= next_row()))
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{
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if (thd->check_killed())
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{
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thd->send_kill_message();
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(void) end_scan();
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DBUG_RETURN(-1);
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}
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/* Check if we can accept the row */
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if (!having || having->val_bool())
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{
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if (store_data_in_temp_table)
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{
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if ((err= table->file->ha_write_tmp_row(table->record[0])))
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{
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bool is_duplicate;
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if (!table->file->is_fatal_error(err, HA_CHECK_DUP))
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continue; // Distinct elimination
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if (create_internal_tmp_table_from_heap(thd, table,
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join->tmp_table_param.
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start_recinfo,
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&join->tmp_table_param.
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recinfo,
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err, 1, &is_duplicate))
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DBUG_RETURN(1);
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if (is_duplicate)
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continue;
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}
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}
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else
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{
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if (join->do_send_rows)
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{
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int error;
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/* result < 0 if row was not accepted and should not be counted */
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if ((error= join->result->send_data(*join->fields)))
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{
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(void) end_scan();
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DBUG_RETURN(error < 0 ? 0 : -1);
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}
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}
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}
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/* limit handling */
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if (++join->send_records >= max_limit && join->do_send_rows)
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{
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if (!(join->select_options & OPTION_FOUND_ROWS))
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break; // LIMIT reached
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join->do_send_rows= 0; // Calculate FOUND_ROWS()
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if (reset_limit)
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*reset_limit= HA_POS_ERROR;
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if (reset_item)
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*reset_item= 0;
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}
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}
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}
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if (err != 0 && err != HA_ERR_END_OF_FILE)
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goto error;
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if ((err= end_scan()))
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goto error_2;
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if (!store_data_in_temp_table && join->result->send_eof())
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DBUG_RETURN(1); // Don't send error to client
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DBUG_RETURN(0);
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error:
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(void) end_scan();
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error_2:
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print_error(err, MYF(0));
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DBUG_RETURN(-1); // Error not sent to client
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}
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void group_by_handler::print_error(int error, myf errflag)
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{
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my_error(ER_GET_ERRNO, MYF(0), error, hton_name(ht)->str);
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}

‎sql/group_by_handler.h‎

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@@ -0,0 +1,133 @@
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/*
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Copyright (c) 2014, SkySQL Ab & MariaDB Foundation
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; version 2 of the License.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
14+
along with this program; if not, write to the Free Software
15+
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
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/*
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This file implements the group_by_handler interface. This interface
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can be used by storage handlers that can intercept summary or GROUP
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BY queries from MariaDB and itself return the result to the user or
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upper level.
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Both main and sub queries are supported. Here are some examples of what the
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storage engine could intersept:
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SELECT count(*) FROM t1;
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SELECT a,count(*) FROM t1 group by a;
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SELECT a,count(*) as sum FROM t1 where b > 10 group by a, order by sum;
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SELECT a,count(*) FROM t1,t2;
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SELECT a, (select sum(*) from t2 where t1.a=t2.a) from t2;
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See https://mariadb.atlassian.net/browse/MDEV-6080 for more information.
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*/
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class JOIN;
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class group_by_handler
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{
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public:
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/* Arguments for group_by_handler, for usage later */
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THD *thd;
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SELECT_LEX *select_lex;
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List<Item> *fields;
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TABLE_LIST *table_list;
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ORDER *group_by, *order_by;
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Item *where, *having;
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handlerton *ht; /* storage engine of this handler */
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/*
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Bit's of things the storage engine can do for this query.
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Should be initialized on object creation.
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*/
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/* Temporary table where all results should be stored in record[0] */
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TABLE *table;
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bool store_data_in_temp_table; /* Set by mariadb */
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group_by_handler(THD *thd_arg, SELECT_LEX *select_lex_arg,
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List<Item> *fields_arg,
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TABLE_LIST *table_list_arg, ORDER *group_by_arg,
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ORDER *order_by_arg, Item *where_arg,
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Item *having_arg, handlerton *ht_arg)
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: thd(thd_arg), select_lex(select_lex_arg), fields(fields_arg),
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table_list(table_list_arg), group_by(group_by_arg),
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order_by(order_by_arg), where(where_arg), having(having_arg),
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ht(ht_arg), table(0), store_data_in_temp_table(0)
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{}
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virtual ~group_by_handler() {}
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/*
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Store pointer to temporary table and objects modified to point to
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the temporary table. This will happen during the optimize phase.
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We provide new 'having' and 'order_by' elements here. The differ from the
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original ones in that these are modified to point to fields in the
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temporary table 'table'.
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Return 1 if the storage handler cannot handle the GROUP BY after all,
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in which case we have to give an error to the end user for the query.
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This is becasue we can't revert back the old having and order_by elements.
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*/
82+
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virtual bool init(TABLE *temporary_table, Item *having_arg,
84+
ORDER *order_by_arg)
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{
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table= temporary_table;
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having= having_arg;
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order_by= order_by_arg;
89+
return 0;
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}
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/*
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Result data is sorted by the storage engine according to order_by (if it
94+
exists) else according to the group_by. If this is not specified,
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MariaDB will store the result set into the temporary table and sort the
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result.
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*/
98+
#define GROUP_BY_ORDER_BY 1
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/* The storage engine can handle DISTINCT */
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#define GROUP_BY_DISTINCT 2
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virtual uint flags() { return 0; }
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/*
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Functions to scan data. All these returns 0 if ok, error code in case
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of error
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*/
107+
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/*
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Initialize group_by scan, prepare for next_row().
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If this is a sub query with group by, this can be called many times for
111+
a query.
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*/
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virtual int init_scan()= 0;
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/*
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Return next group by result in table->record[0].
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Return 0 if row found, HA_ERR_END_OF_FILE if last row and other error
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number in case of fatal error.
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*/
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virtual int next_row()= 0;
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/* End scanning */
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virtual int end_scan()=0;
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/* Information for optimizer (used by EXPLAIN) */
126+
virtual int info(uint flag, ha_statistics *stats)= 0;
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/* Function that calls the above scan functions */
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int execute(JOIN *join);
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/* Report errors */
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virtual void print_error(int error, myf errflag);
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};

‎sql/handler.h‎

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Original file line numberDiff line numberDiff line change
@@ -949,6 +949,10 @@ struct handler_iterator {
949949
};
950950

951951
class handler;
952+
class group_by_handler;
953+
typedef class st_select_lex SELECT_LEX;
954+
typedef struct st_order ORDER;
955+
952956
/*
953957
handlerton is a singleton structure - one instance per storage engine -
954958
to provide access to storage engine functionality that works on the
@@ -1251,6 +1255,24 @@ struct handlerton
12511255
*/
12521256
const char **tablefile_extensions; // by default - empty list
12531257

1258+
/**********************************************************************
1259+
Functions to intercept queries
1260+
**********************************************************************/
1261+
1262+
/*
1263+
Create and return a group_by_handler, if the storage engine can execute
1264+
the summary / group by query.
1265+
If the storage engine can't do that, return NULL.
1266+
1267+
This is only called for SELECT's where all tables are from the same
1268+
storage engine.
1269+
*/
1270+
group_by_handler *(*create_group_by)(THD *thd, SELECT_LEX *select_lex,
1271+
List<Item> *fields,
1272+
TABLE_LIST *table_list, ORDER *group_by,
1273+
ORDER *order_by, Item *where,
1274+
Item *having);
1275+
12541276
/*********************************************************************
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Table discovery API.
12561278
It allows the server to "discover" tables that exist in the storage
@@ -4080,6 +4102,7 @@ class handler :public Sql_alloc
40804102
};
40814103

40824104
#include "multi_range_read.h"
4105+
#include "group_by_handler.h"
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40844107
bool key_uses_partial_cols(TABLE_SHARE *table, uint keyno);
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