background
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Last article GDB debugging Mysql practice (3) priority queue sorting algorithm research (1) The reason why the row UU size in Experiment 3 is 24 is analyzed. In other experiments, the row size was 36, and the number of scanning lines was not as expected. Let's explore this article.
Take experiment 1 as an example
select `aid`,sum(`pv`) as num from article_rank force index(idx_day_aid_pv) where `day`>20190115 group by aid order by num desc LIMIT 10;
The results of optimizer ﹣ trace.join ﹣ execution.steps are as follows
{ "join_execution": { "select#": 1, "steps": [ { "creating_tmp_table": { "tmp_table_info": { "table": "intermediate_tmp_table", "row_length": 20, "key_length": 4, "unique_constraint": false, "location": "memory (heap)", "row_limit_estimate": 838860 } } }, { "converting_tmp_table_to_ondisk": { "cause": "memory_table_size_exceeded", "tmp_table_info": { "table": "intermediate_tmp_table", "row_length": 20, "key_length": 4, "unique_constraint": false, "location": "disk (InnoDB)", "record_format": "fixed" } } }, { "filesort_information": [ { "direction": "desc", "table": "intermediate_tmp_table", "field": "num" } ], "filesort_priority_queue_optimization": { "limit": 10, "rows_estimate": 1057, "row_size": 36, "memory_available": 262144, "chosen": true }, "filesort_execution": [ ], "filesort_summary": { "rows": 11, "examined_rows": 649091, "number_of_tmp_files": 0, "sort_buffer_size": 488, "sort_mode": "<sort_key, additional_fields>" } } ] } }
Why is row ABCD size 36
(gdb) b Sort_param::init_for_filesort Breakpoint 1 at 0xf1a89f: file /root/newdb/mysql-server/sql/filesort.cc, line 107.
(gdb) b Filesort::get_addon_fields Breakpoint 2 at 0xf21231: file /root/newdb/mysql-server/sql/filesort.cc, line 2459. (gdb) b /root/newdb/mysql-server/sql/filesort.cc:2496 Breakpoint 3 at 0xf212f9: file /root/newdb/mysql-server/sql/filesort.cc, line 2496. (gdb) b /root/newdb/mysql-server/sql/filesort.cc:2523 Breakpoint 4 at 0xf2145f: file /root/newdb/mysql-server/sql/filesort.cc, line 2523.
The sorting field is the same as experiment 3, which is 16 bytes. The last 20 bytes are two fields plus 20 bytes + (null fields + 7) / 8, which can be empty. So the last one is 36.
Why rows_estimate is 1057
(gdb) b /root/newdb/mysql-server/sql/filesort.cc:320 Breakpoint 5 at 0xf1b1d9: file /root/newdb/mysql-server/sql/filesort.cc, line 320. ... Breakpoint 5, filesort (thd=0x7f0214000d80, filesort=0x7f021401f668, sort_positions=false, examined_rows=0x7f022804d050, found_rows=0x7f022804d048, returned_rows=0x7f022804d040) at /root/newdb/mysql-server/sql/filesort.cc:320 320 num_rows= table->file->estimate_rows_upper_bound(); (gdb) s ha_innobase::estimate_rows_upper_bound (this=0x7f0214022b50) at /root/newdb/mysql-server/storage/innobase/handler/ha_innodb.cc:13655 ha_innobase::estimate_rows_upper_bound (this=0x7f0214022b50) at /root/newdb/mysql-server/storage/innobase/handler/ha_innodb.cc:13655 warning: Source file is more recent than executable. 13655 DBUG_ENTER("estimate_rows_upper_bound"); (gdb) n 13661 update_thd(ha_thd()); (gdb) n 13663 TrxInInnoDB trx_in_innodb(m_prebuilt->trx); (gdb) n 13665 m_prebuilt->trx->op_info = "calculating upper bound for table rows"; (gdb) n 13667 index = dict_table_get_first_index(m_prebuilt->table); (gdb) n 13669 ulint stat_n_leaf_pages = index->stat_n_leaf_pages; (gdb) p stat_n_leaf_pages $19 = 139646902217632 (gdb) n 13671 ut_a(stat_n_leaf_pages > 0); (gdb) p UNIV_PAGE_SIZE No symbol "UNIV_PAGE_SIZE" in current context. (gdb) n 13674 ((ulonglong) stat_n_leaf_pages) * UNIV_PAGE_SIZE; (gdb) n 13681 estimate = 2 * local_data_file_length (gdb) p local_data_file_length $20 = 16384 (gdb) p stat_n_leaf_pages $21 = 1 (gdb) n 13682 / dict_index_calc_min_rec_len(index); (gdb) n 13684 m_prebuilt->trx->op_info = ""; (gdb) p estimate $22 = 1057 (gdb) p dict_index_calc_min_rec_len(index) $23 = 31
That is to say, the local data file length is 16 bytes, which is a memory page size of the current system.
Calculate the minimum record length in an index
The value of Dict ﹣ index ﹣ calc ﹣ min ﹣ rec ﹣ len (index) is 31. It's too complicated to see it first. It's silly.