Query plan for views and WHERE clauses, Luke is not using the index

Hi all,

I have come across a unexpected behavior.
You can see full detail on an issue on the QGEP project in Github :
https://github.com/QGEP/QGEP/issues/308#issuecomment-323122514

Basically, we have this view with some LEFT JOIN :
http://paste.debian.net/982003/

We have indexes on some fields ( foreign keys, and a GIST index for the
PostGIS geometry field)
If I use the raw SQL defining the view, and add a WHERE clause like:

WHERE "progression_geometry" &&
st_makeenvelope(1728327.03249295568093657,8240789.26074041239917278,1728608.10987572139129043,8240958.16933418624103069,3949)

the query plan is "as expected", as it is using the spatial index (and
others too). This query gets 100 lines from a "main" table containing 20000
lines (and child tables having more). It is pretty fast and "low cost"
See the query plan:
https://explain.depesz.com/s/6Qgb

When we call the WHERE on the view:

EXPLAIN (ANALYZE, COSTS, VERBOSE, BUFFERS)
SELECT *
FROM "qgep"."vw_qgep_reach"
WHERE "progression_geometry" &&
st_makeenvelope(1728327.03249295568093657,8240789.26074041239917278,1728608.10987572139129043,8240958.16933418624103069,3949)

The query plan is "wrong", as PostgreSQL seems to consider it should do a
seq scan on the tables, and only afterwards filter with the WHERE:
https://explain.depesz.com/s/wXV

The query takes about 1 second instead of less than 100ms.

Do you have any hint on this kind of issue ?

Thanks in advance

Regards,

Michaël