Submitted by: @pavel-zotov

Following was done each run on empty database, page = 4k, FW = ON.

DDL:

create or alter view vmon as
select r.mon$record_seq_reads seq_reads,r.mon$record_idx_reads idx_reads
from mon$attachments a left join mon$record_stats r on a.mon$stat_id=r.mon$stat_id
where a.mon$attachment_id = current_connection;
commit;
recreate table test(x int);
commit;
insert into test select rand()*100 from (select 1 i from rdb$types rows 100) a, (select 1 i from rdb$types rows 100) b;
commit;
create index test_x on test(x);
commit; ------------------------------- [1]

TEST-1. Without trace.

select * from vmon; ----------------------- [a]
out nul; select min(x) from test; out;
commit;
select * from vmon; ----------------------- [b]

Result in 2.5:
~~~~~~~~~~

SEQ_READS IDX_READS
========= =====================
10173 382

SEQ_READS IDX_READS
========= =====================
10173 385

Difference of values in IDX_READS column is 385 - 382 = 3 rather than expected 1 (one).
The same result will be for: select x from test order by x rows 1;

Result in 3.0:
~~~~~~~~~~
SEQ_READS IDX_READS
========= =====================
10178 355

SEQ_READS IDX_READS
========= =====================
10178 357

Difference of values in IDX_READS column is 357 - 355 = 2 rather than expected 1 (one).
The same result will be for: select x from test order by x rows 1;

TEST-2. WITH trace launching after "commit; --------- [1]" and before first subsequent query to view "vmon".

select * from vmon; ----------------------- [a] -- this should be issued AFTER trace session will launched in another window
out nul; select min(x) from test; out;
commit;
select * from vmon; ----------------------- [b]

Result in 2.5:
~~~~~~~~~~
SEQ_READS IDX_READS
========== =====================
10173 382

SEQ_READS IDX_READS
========== =====================
10173 386

Difference of values in IDX_READS column is 386 - 382 = 4 rather than expected 1 (one). And this value differ from 3 that was received before, without trace.

Result in 3.0: the same as before, difference is 2 rather than 1. Trace has no affect on this.
~~~~~~~~~~

Q1: why differences in IDX_READS column never was 1 ?
Q2: how TRACE session could affect on result of gathering statistics in MON$RECORD_STATS in 2.5 ?!

PS. Tested on: WI-V2.5.5.26870 and WI-T3.0.0.31836, both - SuperClassic.