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.

mysql-test/include/log_slow_grep.inc

@@ -22,7 +22,7 @@
 
 # InnoDB/Engines
 --let log_expected_matches = $log_slow_verbosity_innodb_expected_matches
---let grep_pattern = ^# Pages_accessed: \d+  Pages_read: \d+  Pages_updated: \d+  Old_rows_read: \d+\$
+--let grep_pattern = ^# Pages_accessed: \d+  Pages_read: \d+  Pages_prefetched: \d+  Pages_updated: \d+  Old_rows_read: \d+\$
 --source include/log_grep.inc
 
 --let grep_pattern = ^# Pages_read_time: \d+\.\d+  Engine_time: \d+\.\d+\$

mysql-test/main/analyze_stmt_prefetch_count.opt

+--innodb-buffer-pool-size=32M --innodb_buffer_pool_dump_at_shutdown=off --innodb_buffer_pool_load_at_startup=off --innodb-stats-persistent=1 --innodb-stats-auto-recalc=off

mysql-test/main/analyze_stmt_prefetch_count.result

+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;

mysql-test/main/analyze_stmt_prefetch_count.test

+--source include/have_innodb.inc
+--source include/have_sequence.inc
+
+
+# Each row is 1K.
+create table t1 (
+  a varchar(255),
+  b varchar(255),
+  c varchar(255),
+  d varchar(255),
+  primary key(a,b,c,d)
+) engine=innodb;
+
+# The data size is 16K * 1K = 16M 
+#  16M / (page_size=16K) = 1K pages.
+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;
+
+--echo # Restart the server to make sure we have an empty InnoDB Buffer Pool 
+--echo # (in the test's .opt file we've disabled buffer pool saving/loading 
+--echo #  and also tried to disable any background activity)
+SET GLOBAL innodb_fast_shutdown=0;
+--source include/restart_mysqld.inc
+
+set @innodb_pages_read0= 
+  (select variable_value 
+   from information_schema.session_status 
+   where variable_name like 'innodb_pages_read');
+
+let $analyze_output= `analyze format=json 
+select * from t1 force index (PRIMARY) order by a,b,c,d`;
+evalp set @js='$analyze_output';
+
+set @js=json_extract(@js, '$.query_block.table.r_engine_stats');
+#select @js;
+set @pages_accessed= cast(json_value(@js,'$.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;
+
+set @total_read = (@pages_read_count + @pages_prefetch_read_count);
+
+select @pages_accessed*0.75 < @total_read, @total_read < @pages_accessed*1.25;
+
+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;
+
+let $analyze_output= `analyze format=json 
+select * from t1 force index (PRIMARY) order by a,b,c,d`;
+evalp set @js='$analyze_output';
+set @js=json_extract(@js, '$.query_block.table.r_engine_stats');
+
+--echo # This must just print pages_accessed. No page reads or prefetch reads,
+--echo # because the previous query has read all the needed pages into the
+--echo # buffer pool, which is set to be large enough to accomodate the whole
+--echo # table.
+--replace_regex /[0-9]+/NUMBER/
+select @js;
+set @pages_accessed2= cast(json_value(@js,'$.pages_accessed') as INT);
+
+select @pages_accessed2 = @pages_accessed;
+drop table t1;

mysql-test/main/log_slow_innodb.result

@@ -23,7 +23,7 @@ UPDATE t1 set b=b+1 where a=1 or a=999;
 [log_grep.inc] lines:   0
 [log_grep.inc] file: log_slow_innodb-verbosity_1 pattern: ^# Tmp_tables: \d+  Tmp_disk_tables: \d+$
 [log_grep.inc] lines:   0
-[log_grep.inc] file: log_slow_innodb-verbosity_1 pattern: ^# Pages_accessed: \d+  Pages_read: \d+  Pages_updated: \d+  Old_rows_read: \d+$ expected_matches: 2
+[log_grep.inc] file: log_slow_innodb-verbosity_1 pattern: ^# Pages_accessed: \d+  Pages_read: \d+  Pages_prefetched: \d+  Pages_updated: \d+  Old_rows_read: \d+$ expected_matches: 2
 [log_grep.inc] found expected match count: 2
 [log_grep.inc] file: log_slow_innodb-verbosity_1 pattern: ^# Pages_read_time: \d+\.\d+  Engine_time: \d+\.\d+$ expected_matches: 2
 [log_grep.inc] found expected match count: 2
@@ -47,7 +47,7 @@ SELECT 1;
 [log_grep.inc] lines:   0
 [log_grep.inc] file: log_slow_innodb-verbosity_2 pattern: ^# Tmp_tables: \d+  Tmp_disk_tables: \d+$
 [log_grep.inc] lines:   0
-[log_grep.inc] file: log_slow_innodb-verbosity_2 pattern: ^# Pages_accessed: \d+  Pages_read: \d+  Pages_updated: \d+  Old_rows_read: \d+$
+[log_grep.inc] file: log_slow_innodb-verbosity_2 pattern: ^# Pages_accessed: \d+  Pages_read: \d+  Pages_prefetched: \d+  Pages_updated: \d+  Old_rows_read: \d+$
 [log_grep.inc] lines:   0
 [log_grep.inc] file: log_slow_innodb-verbosity_2 pattern: ^# Pages_read_time: \d+\.\d+  Engine_time: \d+\.\d+$
 [log_grep.inc] lines:   0
@@ -85,7 +85,7 @@ INSERT INTO t1 VALUE(1000) pattern: ^# Tmp_tables: \d+  Tmp_disk_tables: \d+$
 [log_grep.inc] lines:   0
 [log_grep.inc] file: log_slow_innodb-verbosity_3
 --source include/log_slow_start.inc
-INSERT INTO t1 VALUE(1000) pattern: ^# Pages_accessed: \d+  Pages_read: \d+  Pages_updated: \d+  Old_rows_read: \d+$
+INSERT INTO t1 VALUE(1000) pattern: ^# Pages_accessed: \d+  Pages_read: \d+  Pages_prefetched: \d+  Pages_updated: \d+  Old_rows_read: \d+$
 [log_grep.inc] lines:   0
 [log_grep.inc] file: log_slow_innodb-verbosity_3
 --source include/log_slow_start.inc

sql/ha_handler_stats.h

@@ -30,6 +30,15 @@ class ha_handler_stats
   /* Time spent reading pages, in timer_tracker_frequency() units */
   ulonglong pages_read_time;
 
+  /* 
+    Number of pages that we've requested to prefetch while running the query.
+    Note that we don't know:
+    - how much time was spent reading these pages (and how to count the time
+      if reading was done in parallel)
+    - whether the pages were read by "us" or somebody else...
+  */
+  ulonglong pages_prefetched;
+
   ulonglong undo_records_read;
 
   /* Time spent in engine, in timer_tracker_frequency() units */

sql/log.cc

@@ -3283,10 +3283,12 @@ bool MYSQL_QUERY_LOG::write(THD *thd, time_t current_time,
 
       if (my_b_printf(&log_file,
                       "# Pages_accessed: %lu  Pages_read: %lu  "
+                      "Pages_prefetched: %lu  "
                       "Pages_updated: %lu  Old_rows_read: %lu\n"
                       "# Pages_read_time: %s  Engine_time: %s\n",
                       (ulong) stats->pages_accessed,
                       (ulong) stats->pages_read_count,
+                      (ulong) stats->pages_prefetched,
                       (ulong) stats->pages_updated,
                       (ulong) stats->undo_records_read,
                       query_time_buff, lock_time_buff))

sql/sql_explain.cc

@@ -1727,6 +1727,8 @@ static void trace_engine_stats(handler *file, Json_writer *writer)
     if (hs->pages_read_time)
       writer->add_member("pages_read_time_ms").
         add_double(hs->pages_read_time * 1000. / timer_tracker_frequency());
+    if (hs->pages_prefetched)
+      writer->add_member("pages_prefetch_read_count").add_ull(hs->pages_prefetched);
     if (hs->undo_records_read)
       writer->add_member("old_rows_read").add_ull(hs->undo_records_read);
     writer->end_object();

storage/innobase/buf/buf0rea.cc

@@ -427,6 +427,7 @@ buf_read_ahead_random(const page_id_t page_id, ulint zip_size, bool ibuf)
 
   if (count)
   {
+    mariadb_increment_pages_prefetched(count);
     DBUG_PRINT("ib_buf", ("random read-ahead %zu pages from %s: %u",
 			  count, space->chain.start->name,
 			  low.page_no()));
@@ -671,6 +672,7 @@ buf_read_ahead_linear(const page_id_t page_id, ulint zip_size, bool ibuf)
 
   if (count)
   {
+    mariadb_increment_pages_prefetched(count);
     DBUG_PRINT("ib_buf", ("random read-ahead %zu pages from %s: %u",
                           count, space->chain.start->name,
                           new_low.page_no()));

storage/innobase/include/mariadb_stats.h

@@ -79,6 +79,12 @@ inline void mariadb_increment_undo_records_read()
     stats->undo_records_read++;
 }
 
+inline void mariadb_increment_pages_prefetched(ulint n_pages)
+{
+  if (ha_handler_stats *stats= mariadb_stats)
+    stats->pages_prefetched += n_pages;
+}
+
 /*
   The following has to be identical code as measure() in sql_analyze_stmt.h