3581 Marko Makela 2011-08-10
Bug#12835650 VARCHAR maximum length performance impact
row_sel_field_store_in_mysql_format(): Do not pad the unused part of
the buffer reserved for a True VARCHAR column (introduced in 5.0.3).
Add Valgrind instrumentation ensuring that the unused part will be
flagged uninitialized.
Before this, buffers which were used to send VARCHARs from InnoDB to the MySQL server were padded with 0s if the string was shorter than specified by the column. If, e.g., the string "foo" was stored in a VARCHAR(8), InnoDB used to write "3foo00000" to the buffer (the first character - 3 - determines the actual length of the string). However, even though these trailing bytes are not used anywhere, writing 0s to the buffer certainly has a cost. Hence the fix which stops InnoDB from writing them.
Oh my were we up for a ride! All of a sudden Valgrind started barking like this (and similar) for a number of tests:
Thread 19:The modifications we did to the server to remedy the problems can be divided into two categories as described below. In the end, the result was significantly improved performance.
Syscall param pwrite64(buf) points to uninitialised byte(s)
at 0x381C60EEE3: ??? (in /lib64/libpthread-2.12.so)
by 0x84B703: my_pwrite (my_pread.c:162) by 0x86FE6F: mi_nommap_pwrite (mysql_file.h:1203)
by 0x88621B: _mi_write_static_record (mi_statrec.c:65)
by 0x889592: mi_write (mi_write.c:142)
by 0x532085: handler::ha_write_row(unsigned char*) (handler.cc:6043)
by 0x69B7F7: end_write(JOIN*, st_join_table*, bool) (sql_select.cc:20237)
by 0x69A43C: evaluate_join_record(JOIN*, st_join_table*, int)
(sql_select.cc:19187)
by 0x69A8E0: sub_select(JOIN*, st_join_table*, bool) (sql_select.cc:19294)
