BUG #16112: large, unexpected memory consumption

The following bug has been logged on the website:

Bug reference:      16112
Logged by:          Ben Cornett
Email address:      ben@lantern.is
PostgreSQL version: 12.0
Operating system:   linux 2.6.32
Description:

Creation of table t1 in the query below causes the server process to consume
close to 1GB of memory.  The amount of memory consumed is proportional to
the value passed to generate_series in the first query.

CREATE temp TABLE t0 AS
SELECT
        i,
        ARRAY[1,2,3] a
FROM GENERATE_SERIES(1, 12000000) i
;

CREATE TEMP TABLE t1 AS
SELECT
        i,
        x
FROM t0, UNNEST(a) x;

I observed the same behavior in other test queries that included implicit
lateral joins.

Thanks very much,
Ben