Description:
Manual (http://dev.mysql.com/doc/refman/5.7/en/kill.html) says:
"During ALTER TABLE operations, the kill flag is checked before each block of rows are read from the original table. If the kill flag was set, the statement is aborted and the temporary table is deleted."
So, it mentions "block of rows" and prompts users to ask questions like:
"What is the size of that block?", or
"What constitutes the block of rows?"
There is no answer in the manual, neither in general nor for specific storage engine.
How to repeat:
Search the source code related to ALTER TABLE for the thd->killed flag checks. On current 5.7 code I see:
[openxs@fc23 mysql-server]$ grep -n 'thd->kill' sql/sql_table.cc
2442: if (thd->killed)
10115: if (thd->killed)
10341: if (thd->killed)
[openxs@fc23 mysql-server]$
The first hit is in the mysql_rm_table_no_locks() function:
/**
Execute the drop of a normal or temporary table.
@param thd Thread handler
@param tables Tables to drop
@param if_exists If set, don't give an error if table doesn't exists.
In this case we give an warning of level 'NOTE'
@param drop_temporary Only drop temporary tables
@param drop_view Allow to delete VIEW .frm
@param dont_log_query Don't write query to log files. This will also not
generate warnings if the handler files doesn't exists
@retval 0 ok
@retval 1 Error
@retval -1 Thread was killed
...
int mysql_rm_table_no_locks(THD *thd, TABLE_LIST *tables, bool if_exists,
bool drop_temporary, bool drop_view,
bool dont_log_query)
...
The second hit is in the copy_data_between_tables() function that is really executed when ALTER does a table copy (and reads rows):
static int
copy_data_between_tables(PSI_stage_progress *psi,
TABLE *from,TABLE *to,
List<Create_field> &create,
ha_rows *copied,
ha_rows *deleted,
Alter_info::enum_enable_or_disable keys_onoff,
Alter_table_ctx *alter_ctx)
{
...
The code is like this:
10113 while (!(error=info.read_record(&info)))
10114 {
10115 if (thd->killed)
10116 {
10117 thd->send_kill_message();
10118 error= 1;
10119 break;
10120 }
10121 /* Return error if source table isn't empty. */
10122 if (alter_ctx->error_if_not_empty)
10123 {
10124 error= 1;
10125 break;
10126 }
...
10182 thd->get_stmt_da()->inc_current_row_for_condition();
10183 }
10184 end_read_record(&info);
and tracing in gdb shows that at least for InnoDB table the loop is executed for each row in the original table ALTERed, and on each iteration we start from checking the kill flag!
The third hit is in mysql_checksum_table() function:
bool mysql_checksum_table(THD *thd, TABLE_LIST *tables,
HA_CHECK_OPT *check_opt)
...
so is not directly relevant.
Suggested fix:
Document clearly that when ALTER requires a table copy it checkes kill flag before reading every next row from the table, or otherwise explain how read_record() function is implemented for different storage engines available in MySQL.
