Function approximate_row_count returns 0 for caggs (#6053)

The approximate_row_count function is executed directly on the user view
instead of corresponding materialized hypertable which returns 0 for
caggs. The function is updated to fetch the details for materialized
hypertable information corresponding to cagg and then get the
approximate_row_count for the materialized hypertable.

Fixes #6051

.unreleased/bugfix_6053

@@ -0,0 +1 @@
+Fixes: #6053 Function approximate_row_count returns 0 for caggs

sql/size_utils.sql

@@ -415,6 +415,7 @@ RETURNS BIGINT
 LANGUAGE PLPGSQL VOLATILE STRICT AS
 $BODY$
 DECLARE
+    mat_ht           REGCLASS = NULL;
     local_table_name       NAME = NULL;
     local_schema_name      NAME = NULL;
     is_distributed   BOOL = FALSE;
@@ -428,6 +429,20 @@ DECLARE
     max_compressed_row_count BIGINT = 1000;
     is_compressed_chunk INTEGER;
 BEGIN
+    -- Check if input relation is continuous aggregate view then
+    -- get the corresponding materialized hypertable and schema name
+    SELECT format('%I.%I', ht.schema_name, ht.table_name)::regclass
+    INTO mat_ht
+    FROM pg_class c
+    JOIN pg_namespace n ON (n.OID = c.relnamespace)
+    JOIN _timescaledb_catalog.continuous_agg a ON (a.user_view_schema = n.nspname AND a.user_view_name = c.relname)
+    JOIN _timescaledb_catalog.hypertable ht ON (a.mat_hypertable_id = ht.id)
+    WHERE c.OID = relation;
+
+    IF mat_ht IS NOT NULL THEN
+        relation = mat_ht;
+    END IF;
+
     SELECT relname, nspname FROM pg_class c
     INNER JOIN pg_namespace n ON (n.OID = c.relnamespace)
     INTO local_table_name, local_schema_name

tsl/test/expected/size_utils_tsl.out

@@ -52,6 +52,12 @@ SELECT * FROM chunks_detailed_size('hypersize_cagg') ORDER BY node_name;
 --------------+------------+-------------+-------------+-------------+-------------+-----------
 (0 rows)
 
+SELECT * FROM approximate_row_count('hypersize_cagg');
+ approximate_row_count 
+-----------------------
+                     0
+(1 row)
+
 -- Test size functions on non-empty countinuous aggregate
 CALL refresh_continuous_aggregate('hypersize_cagg', NULL, NULL);
 SELECT * FROM hypertable_size('hypersize_cagg');
@@ -90,3 +96,10 @@ SELECT * FROM chunks_detailed_size(:'MAT_HYPERTABLE_NAME') ORDER BY node_name;
  _timescaledb_internal | _hyper_2_2_chunk |        8192 |       16384 |        8192 |       32768 | 
 (1 row)
 
+ANALYZE :MAT_HYPERTABLE_NAME;
+SELECT * FROM approximate_row_count('hypersize_cagg');
+ approximate_row_count 
+-----------------------
+                     1
+(1 row)
+

tsl/test/sql/size_utils_tsl.sql

@@ -21,6 +21,7 @@ SELECT * FROM chunks_detailed_size(:'MAT_HYPERTABLE_NAME') ORDER BY node_name;
 SELECT * FROM hypertable_size('hypersize_cagg');
 SELECT * FROM hypertable_detailed_size('hypersize_cagg') ORDER BY node_name;
 SELECT * FROM chunks_detailed_size('hypersize_cagg') ORDER BY node_name;
+SELECT * FROM approximate_row_count('hypersize_cagg');
 
 -- Test size functions on non-empty countinuous aggregate
 CALL refresh_continuous_aggregate('hypersize_cagg', NULL, NULL);
@@ -30,3 +31,5 @@ SELECT * FROM chunks_detailed_size('hypersize_cagg') ORDER BY node_name;
 SELECT * FROM hypertable_size(:'MAT_HYPERTABLE_NAME');
 SELECT * FROM hypertable_detailed_size(:'MAT_HYPERTABLE_NAME') ORDER BY node_name;
 SELECT * FROM chunks_detailed_size(:'MAT_HYPERTABLE_NAME') ORDER BY node_name;
+ANALYZE :MAT_HYPERTABLE_NAME;
+SELECT * FROM approximate_row_count('hypersize_cagg');