As part of the Postgres 8.3 merge, all heap tables now automatically
create an array type. The array type will usually be created with
typname '_<heap_name>' since the automatically created composite type
already takes the typname '<heap_name>' first. If typname
'_<heap_name>' is taken, the logic will continue to prepend
underscores until no collision (truncating the end if typname gets
past NAMEDATALEN of 64). This might be an oversight in upstream
Postgres since certain scenarios involving creating a large number of
heap tables with similar names could result in a lot of typname
collisions until no heap tables with similar names can be
created. This is very noticable in Greenplum heap partition tables
because Greenplum has logic to automatically name child partitions
with similar names instead of having the user name each child
partition.

To prevent typname collision failures when creating a heap partition
table with a large number of child partitions, we will now stop
automatically creating the array type for child partitions.

References:
https://www.postgresql.org/message-id/flat/20070302234016.GF3665%40fetter.org
https://github.com/postgres/postgres/commit/bc8036fc666a8f846b1d4b2f935af7edd90eb5aa