pg_stat_progress_create_index vs. parallel index builds

Hi,

While experimenting with parallel index builds, I've noticed a somewhat 
strange behavior of pg_stat_progress_create_index when a btree index is 
built with parallel workers - some of the phases seem to be missing.

In serial (no parallelism) mode, the progress is roughly this (it's 
always the first/last timestamp of each phase):

              |   command    |             phase
-------------+--------------+----------------------------------------
  12:56:01 AM | CREATE INDEX | building index: scanning table
          ...
  01:06:22 AM | CREATE INDEX | building index: scanning table
  01:06:23 AM | CREATE INDEX | building index: sorting live tuples
          ...
  01:13:10 AM | CREATE INDEX | building index: sorting live tuples
  01:13:11 AM | CREATE INDEX | building index: loading tuples in tree
          ...
  01:24:02 AM | CREATE INDEX | building index: loading tuples in tree

So it goes through three phases:

1) scanning tuples
2) sorting live tuples
3) loading tuples in tree

But with parallel build index build, it changes to:

              |   command    |             phase
-------------+--------------+----------------------------------------
  11:40:48 AM | CREATE INDEX | building index: scanning table
          ...
  11:47:24 AM | CREATE INDEX | building index: scanning table (scan
                complete)
  11:56:22 AM | CREATE INDEX | building index: scanning table
  11:56:23 AM | CREATE INDEX | building index: loading tuples in tree
          ...
  12:05:33 PM | CREATE INDEX | building index: loading tuples in tree

That is, the "sorting live tuples" phase disappeared, and instead it 
seems to be counted in the "scanning table" one, as if there was an 
update of the phase missing.

I've only tried this on master, but I assume it behaves like this in the 
older releases too. I wonder if this is intentional - it sure is a bit 
misleading.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company