On 2021/05/18 18:23, Masahiro Ikeda wrote:
On 2021/05/18 16:01, Fujii Masao wrote:
On 2021/05/18 13:20, Masahiro Ikeda wrote:
Tid Range Scan increments the tup_returned, and
pg_stat_all_tables.seq_tup_read is also incremented. I thought it's ok because
Tid Range Scan is like sequential scan.
Yes, you're right. One interesting thing I found is;
when Tid Range Scan happens, seq_tup_read is incremented
but seq_scan is not. I'm not sure if this is expected behavior or not.
The following comment says that this behavior is expected. But, I agree it's
odd and it's natural both seq_tup_read and seq_scan are incremented at the
same time or not...
/*
* Currently, we only have a stats counter for sequential heap scans (but
* e.g for bitmap scans the underlying bitmap index scans will be counted,
* and for sample scans we update stats for tuple fetches).
*/
if (scan->rs_base.rs_flags & SO_TYPE_SEQSCAN)
pgstat_count_heap_scan(scan->rs_base.rs_rd);
That's the reason why the document of
pg_stat_all_tables.seq_tup_read says "Number of live rows fetched by
sequential scans"
Regarding the original issue, as far as I understand correctly,
* pg_stat_database.tup_returned = sum(pg_stat_all_tables.seq_tup_read) +
sum(pg_stat_all_indexes.idx_tup_read)
* pg_stat_database.tup_fetched = sum(pg_stat_all_tables.idx_tup_fetch)
But the counters for some system catalogs like pg_database shared
across all databases of a cluster are excluded from that calculation.
Is this my understanding right? If right, probably we can reuse
the existing descriptions for those counters to document
pg_stat_database counters. For example,
Yes, my understanding is same now.
pg_stat_database.tup_returned:> Number of live rows fetched by sequential and
index scans in this database
I wonder "live rows fetched by index scans" may mislead. I think "live" means
it's not dead tuple and "rows" mean the tuple user want to get.
But, pg_stat_all_indexes.idx_tup_read says that "index entires returned by
scans on this index". There is no meaning of "live" and "rows", so I thought
it's better to distinguish them.
So, why don't you change to "Number of live rows fetched by sequential scans
and index entries returned by index scans in this database"?
Yes, LGTM.
pg_stat_database.tup_fetched:
Number of index entries returned by scans on indexes in this database
Is this the sum of pg_stat_all_indexes.idx_tup_read? This is accounted to
pg_stat_database.tup_returned.
I was thinking that pg_stat_database.tup_fetched is the same as
the sum of pg_stat_all_tables.idx_tup_fetch. Because they both
are incremented by bitmap index scans, but pg_stat_all_indexes.idx_tup_read
is not.
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
