Re: Add pg_stat_kind_info system view

Hi,

Thanks for working on this. I like this overall, but have some comments.

1/

+ One row for each loaded statistics kind, showing metadata
about the kind.

The description says "showing metadata about the kind" but
the count column returns live runtime data (via pgstat_get_entry_count()),
not just metadata. The description should acknowledge both.

2/

+ <structfield>shared_size</structfield> <type>bigint</type>
+ </para>
+ <para>
+ Size in bytes of a shared memory entry for this statistics kind.
+ <literal>NULL</literal> for built-in fixed-amount kinds, whose
+ single entry lives in a statically-allocated slot rather than a
+ sized shared memory entry.

I think we should just use info->shared_data_len for both fixed and
variable-length kinds. This shows the entry_size which is what an
extension developer will care about. Even info->shared_size for
variable-length kinds does not include the per-entry overhead,
so info->shared_data_len will be useful to understand if the size
of your entry changed after an upgrade, etc. Also, we should
rename it to "entry_size" instead of "shared_size"

3/

+ <structfield>count</structfield> <type>bigint</type>
+ </para>
+ <para>
+ Number of tracked entries for this kind. For fixed-amount
kinds, this is
+ always 1. For variable-numbered kinds, this is the number of objects
+ currently tracked. <literal>NULL</literal> if the kind does not track
+ entry counts.
+ </para>
+ </entry>

I think this should be 0 for all kinds by default. Only ever > 0 for
variable-numbered
kinds with entry tracking enabled. No NULLs. The fields should be called
"entry_count" instead of "count". We should update the documentation to
reflect that as well. That seems easier to query than having mixed NULLs and
numbers.

Sami Imseih
Amazon Web Services (AWS)