I missed it completely, but on 24th of March 2017, Alvaro Herrera committed patch:

Implement multivariate n-distinct coefficients Add support for explicitly declared statistic objects (CREATE STATISTICS), allowing collection of statistics on more complex combinations that individual table columns. Companion commands DROP STATISTICS and ALTER STATISTICS ... OWNER TO / SET SCHEMA / RENAME are added too. All this DDL has been designed so that more statistic types can be added later on, such as multivariate most-common-values and multivariate histograms between columns of a single table, leaving room for permitting columns on multiple tables, too, as well as expressions. This commit only adds support for collection of n-distinct coefficient on user-specified sets of columns in a single table. This is useful to estimate number of distinct groups in GROUP BY and DISTINCT clauses; estimation errors there can cause over-allocation of memory in hashed aggregates, for instance, so it's a worthwhile problem to solve. A new special pseudo-type pg_ndistinct is used. (num-distinct estimation was deemed sufficiently useful by itself that this is worthwhile even if no further statistic types are added immediately; so much so that another version of essentially the same functionality was submitted by Kyotaro Horiguchi: https://postgr.es/m/.173334..horiguchi.kyotaro@lab.ntt.co.jp though this commit does not use that code.) Author: Tomas Vondra. Some code rework by Álvaro. Ideriha Takeshi Discussion: https://postgr.es/m/.4080608@fuzzy.cz https://postgr.es/m/.ixlaueanxegqd5gr@alvherre.pgsql

Afterwards, there were couple more commits related to it:

- On 5th of April 2017, patch committed by Simon Riggs
- On 17th of April 2017, patch committed by Alvaro Herrera
- On 12nd of May 2017, patch committed by Alvaro Herrera

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