mariadb/mysql-test/main/analyze_stmt_prefetch_count.result
Sergei Petrunia 513c827041 MDEV-34190: r_engine_stats.pages_read_count is unrealistically low
The symptoms were: take a server with no activity and a table that's
not in the buffer pool. Run a query that reads the whole table and
observe that r_engine_stats.pages_read_count shows about 2% of the table
was read. Who reads the rest?

The cause was that page prefetching done inside InnoDB was not counted.

This counts page prefetch requests made in buf_read_ahead_random() and
buf_read_ahead_linear() and makes them visible in:

- ANALYZE: r_engine_stats.pages_prefetch_read_count
- Slow Query Log: Pages_prefetched:

This patch intentionally doesn't attempt to count the time to read the
prefetched pages:
* there's no obvious place where one can do it
* prefetch reads may be done in parallel (right?), it is not clear how
  to count the time in this case.
2024-07-04 15:24:49 +03:00

60 lines
2.2 KiB
Text

create table t1 (
a varchar(255),
b varchar(255),
c varchar(255),
d varchar(255),
primary key(a,b,c,d)
) engine=innodb;
SET unique_checks=0, foreign_key_checks= 0;
begin;
insert into t1 select
repeat(uuid(), 7),
repeat(uuid(), 7),
repeat(uuid(), 7),
repeat(uuid(), 7)
from seq_1_to_16384;
insert into t1 values ('z','z','z','z');
commit;
# Restart the server to make sure we have an empty InnoDB Buffer Pool
# (in the test's .opt file we've disabled buffer pool saving/loading
# and also tried to disable any background activity)
SET GLOBAL innodb_fast_shutdown=0;
# restart
set @innodb_pages_read0=
(select variable_value
from information_schema.session_status
where variable_name like 'innodb_pages_read');
set @js='$analyze_output';
set @js=json_extract(@js, '$.query_block.table.r_engine_stats');
set @pages_accessed= cast(json_value(@js,'$.pages_accessed') as INT);
set @pages_read_count= cast(json_value(@js,'$.pages_read_count') as INT);
set @pages_prefetch_read_count= cast(json_value(@js,'$.pages_prefetch_read_count') as INT);
select @pages_accessed > 1000 and @pages_accessed < 1500;
@pages_accessed > 1000 and @pages_accessed < 1500
1
set @total_read = (@pages_read_count + @pages_prefetch_read_count);
select @pages_accessed*0.75 < @total_read, @total_read < @pages_accessed*1.25;
@pages_accessed*0.75 < @total_read @total_read < @pages_accessed*1.25
1 1
set @innodb_pages_read1=
(select variable_value
from information_schema.session_status
where variable_name like 'innodb_pages_read');
set @innodb_pages_read_incr= (@innodb_pages_read1 - @innodb_pages_read0);
select @innodb_pages_read_incr > 1000, @innodb_pages_read_incr < 1500;
@innodb_pages_read_incr > 1000 @innodb_pages_read_incr < 1500
1 1
set @js='$analyze_output';
set @js=json_extract(@js, '$.query_block.table.r_engine_stats');
# This must just print pages_accessed. No page reads or prefetch reads,
# because the previous query has read all the needed pages into the
# buffer pool, which is set to be large enough to accomodate the whole
# table.
select @js;
@js
{"pages_accessed": NUMBER}
set @pages_accessed2= cast(json_value(@js,'$.pages_accessed') as INT);
select @pages_accessed2 = @pages_accessed;
@pages_accessed2 = @pages_accessed
1
drop table t1;