Re: pg_stats and range statistics

Hi,

thanks for the review and corrections.

On 11.07.2021 21:54, Soumyadeep Chakraborty wrote:
> Hello,
>
> This should have been added with [1].
>
> Excerpt from the documentation:
> "pg_stats is also designed to present the information in a more readable
> format than the underlying catalog — at the cost that its schema must
> be extended whenever new slot types are defined for pg_statistic." [2]
>
> So, I added a reminder in pg_statistic.h.

Good point.

> Attached is v2 of this patch with some cosmetic changes.

I wonder why "TODO: catalog version bump"? This patch doesn't change
catalog structure, or I miss something?

> Renamed the columns a
> bit and updated the docs to be a bit more descriptive.
> (range_length_empty_frac -> empty_range_frac, range_bounds_histogram ->
> range_bounds_histograms)

I intended to make the same prefix ("range_") for all columns concerned
with range types, although I'm fine with the proposed naming.

> One question:
>
> We do have the option of representing the histogram of lower bounds separately
> from the histogram of upper bounds, as two separate view columns. Don't know if
> there is much utility though and there is a fair bit of added complexity: see
> below. Thoughts?

I thought about it too, and decided not to transform the underlying data
structure. As far as I can see, pg_stats never employed such
transformations. For example, STATISTIC_KIND_DECHIST is an array
containing the histogram followed by the average in its last element. It
is shown in pg_stats.elem_count_histogram as is, although it arguably
may be splitted into two fields. All in all, I believe pg_stats's job is
to "unpack" stavalues and stanumbers into meaningful fields, and not to
try to go deeper than that.

>
> My attempts via SQL (unnest -> lower|upper -> array_agg) were futile given
> unnest does not play nice with anyarray. For instance:
>
> select unnest(stavalues1) from pg_statistic;
> ERROR: cannot determine element type of "anyarray" argument
>
> Maybe the only option is to write a UDF pg_get_{lower|upper}_bounds_histogram
> which can do something similar to what calc_hist_selectivity does:
>
> /*
> * Convert histogram of ranges into histograms of its lower and upper
> * bounds.
> */
> nhist = hslot.nvalues;
> hist_lower = (RangeBound *) palloc(sizeof(RangeBound) * nhist);
> hist_upper = (RangeBound *) palloc(sizeof(RangeBound) * nhist);
> for (i = 0; i < nhist; i++)
> {
> bool empty;
>
> range_deserialize(rng_typcache, DatumGetRangeTypeP(hslot.values[i]),
> &hist_lower[i], &hist_upper[i], &empty);
> /* The histogram should not contain any empty ranges */
> if (empty)
> elog(ERROR, "bounds histogram contains an empty range");
> }
>
> This is looking good and ready.
>
> [1] https://github.com/postgres/postgres/commit/918eee0c497c88260a2e107318843c9b1947bc6f
> [2] https://www.postgresql.org/docs/devel/view-pg-stats.html
>
> Regards,
> Soumyadeep (VMware)