[postgis-users] Queries on partitioned table not behaving as expected

[Rubio Vaughan]

Dear all,

We are trying to load a large LIDAR pointcloud dataset into multiple 
tables in PostGIS, using the point cloud extension. Eventually the whole 
data set will consist of 1400+ tables, containing about 1 million 
records each. Using a union view on all these tables would result in 
terrible performance, which is why we are trying to optimize query 
performance using partitioned tables. According to the documentation, 
the use of partitioned tables with CHECK constraints should cause the 
query planner to only scan those tables for which the CHECK constraint 
matches. Excerpt from the documentation:
--------------------------------------------------------------------------------
*18.7.4. Other Planner Options*
constraint_exclusion(enum): Controls the query planner's use of table 
constraints to optimize queries. The allowed values of 
constraint_exclusion are on (examine constraints for all tables), off 
(never examine constraints), and partition (examine constraints only for 
inheritance child tables and UNION ALL subqueries). partition is the 
default setting. It is often used with inheritance and partitioned 
tables to improve performance. When this parameter allows it for a 
particular table, the planner compares query conditions with the table's 
CHECK constraints, and omits scanning tables for which the conditions 
contradict the constraints.
--------------------------------------------------------------------------------

However, as you can see from the example query below, the indexes for 
all child tables are still scanned. I would expect the query planner to 
only scan table c_25gn1, which contains the queried region. Does anyone 
here have experience with partitioned tables? I would be delighted to 
get some pointers for figuring out this problem.

Thanks in advance for any help!

Best regards,
Rubio Vaughan

*General info:**
*--------------------------------------------------------------------------------
SELECT postgis_full_version();
"POSTGIS="2.2.0 r14208" GEOS="3.5.0-CAPI-1.9.0 r4084" SFCGAL="1.2.0" 
PROJ="Rel. 4.8.0, 6 March 2012" GDAL="GDAL 2.0.1, released 2015/09/15" 
LIBXML="2.9.1" LIBJSON="0.11.99" TOPOLOGY (topology procs from "2.1.4 
r12966" need upgrade) RASTER"

SHOW "constraint_exclusion";
"partition"
--------------------------------------------------------------------------------

*The parent table:*
--------------------------------------------------------------------------------
CREATE TABLE ahn3_pointcloud.ahn3_all
(
id integer NOT NULL,
pa pcpatch(7)
)
--------------------------------------------------------------------------------

*One of the child tables:*
--------------------------------------------------------------------------------
CREATE TABLE ahn3_pointcloud.c_25dn2
(
   id serial NOT NULL,
   pa pcpatch(7),
   CONSTRAINT c_25dn2_pkey PRIMARY KEY (id),
   CONSTRAINT check_extent CHECK (pc_intersects(pa, 
st_geomfromtext('POLYGON((115000 487500,120000 487500,120000 
481250,115000 481250,115000 487500))'::text, 28992)))
)
INHERITS (ahn3_pointcloud.ahn3_all)
--------------------------------------------------------------------------------

*Example query:*
--------------------------------------------------------------------------------
EXPLAIN SELECT COUNT(pa) FROM ahn3_pointcloud.ahn3_all
WHERE PC_Intersects(pa, ST_MakeEnvelope(120740,486076,121074,486292, 28992))

"Aggregate  (cost=301989.36..301989.37 rows=1 width=32)"
"  ->  Append  (cost=0.00..301248.82 rows=296216 width=32)"
"        ->  Seq Scan on ahn3_all  (cost=0.00..0.00 rows=1 width=32)"
"              Filter: 
(('010300002040710000010000000500000000000000407AFD4000000000<snip>
"        ->  Bitmap Heap Scan on c_25gn1 (cost=6064.85..59546.09 
rows=58574 width=32)"
"              Recheck Cond: 
('010300002040710000010000000500000000000000407AFD40000<snip>
"              Filter: 
_st_intersects('010300002040710000010000000500000000000000407<snip>
"              ->  Bitmap Index Scan on c_25gn1_idx (cost=0.00..6050.20 
rows=175722 width=0)"
"                    Index Cond: 
('010300002040710000010000000500000000000000407AFD4<snip>
"        ->  Bitmap Heap Scan on c_30fz1 (cost=7990.39..77907.99 
rows=76576 width=32)"
"              Recheck Cond: 
('010300002040710000010000000500000000000000407AFD40000<snip>
"              Filter: 
_st_intersects('010300002040710000010000000500000000000000407<snip>
"              ->  Bitmap Index Scan on c_30fz1_idx (cost=0.00..7971.24 
rows=229727 width=0)"
"                    Index Cond: 
('010300002040710000010000000500000000000000407AFD4<snip>
"        ->  Bitmap Heap Scan on c_30fz2 (cost=7393.57..71499.52 
rows=70210 width=32)"
"              Recheck Cond: 
('010300002040710000010000000500000000000000407AFD40000<snip>
"              Filter: 
_st_intersects('010300002040710000010000000500000000000000407<snip>
"              ->  Bitmap Index Scan on c_30fz2_idx (cost=0.00..7376.02 
rows=210631 width=0)"
"                    Index Cond: 
('010300002040710000010000000500000000000000407AFD4<snip>
"        ->  Bitmap Heap Scan on c_25ez1 (cost=2624.69..26079.63 
rows=25688 width=32)"
"              Recheck Cond: 
('010300002040710000010000000500000000000000407AFD40000<snip>
"              Filter: 
_st_intersects('010300002040710000010000000500000000000000407<snip>
"              ->  Bitmap Index Scan on c_25ez1_idx (cost=0.00..2618.27 
rows=77064 width=0)"
"                    Index Cond: 
('010300002040710000010000000500000000000000407AFD4<snip>
"        ->  Bitmap Heap Scan on c_25dn2 (cost=6714.83..66215.58 
rows=65167 width=32)"
"              Recheck Cond: 
('010300002040710000010000000500000000000000407AFD40000<snip>
"              Filter: 
_st_intersects('010300002040710000010000000500000000000000407<snip>
"              ->  Bitmap Index Scan on c_25dn2_idx (cost=0.00..6698.54 
rows=195500 width=0)"
"                    Index Cond: 
('010300002040710000010000000500000000000000407AFD4<snip>
--------------------------------------------------------------------------------



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