Adding an "and is not null" on an indexed field slows the query down immensely.

I have this query it runs reasonably quickly.


SELECT "consolidated_urls".* FROM "consolidated_urls" INNER JOIN "topical_urls"
 ON "consolidated_urls".id = "topical_urls".consolidated_url_id
 WHERE (("topical_urls".domain_id = 157) AND ((topical_urls.hidden = 'f')))
  ORDER BY index_delta DESC LIMIT 10

The explain for this goes like this.

"Limit  (cost=29899.43..29899.46 rows=10 width=1880)"
"  ->  Sort  (cost=29899.43..29906.04 rows=2642 width=1880)"
"        Sort Key: consolidated_urls.index_delta"
"        ->  Nested Loop  (cost=101.29..29842.34 rows=2642 width=1880)"
"              ->  Bitmap Heap Scan on topical_urls
(cost=101.29..7490.32 rows=2642 width=4)"
"                    Recheck Cond: (domain_id = 157)"
"                    Filter: (NOT hidden)"
"                    ->  Bitmap Index Scan on
index_topical_urls_on_domain_id_and_consolidated_url_id
(cost=0.00..100.63 rows=2643 width=0)"
"                          Index Cond: (domain_id = 157)"
"              ->  Index Scan using consolidated_urls_pkey on
consolidated_urls  (cost=0.00..8.45 rows=1 width=1880)"
"                    Index Cond: (consolidated_urls.id =
topical_urls.consolidated_url_id)"


I add one more clause on to it to filter out index_deltas that are not
null and the query becomes unusably slow.

SELECT "consolidated_urls".* FROM "consolidated_urls" INNER JOIN "topical_urls"
 ON "consolidated_urls".id = "topical_urls".consolidated_url_id
 WHERE (("topical_urls".domain_id = 157) AND ((topical_urls.hidden = 'f')))
 AND (index_delta IS NOT NULL) ORDER BY index_delta DESC LIMIT 10

The explain for this is goes like this

"Limit  (cost=0.00..20555.33 rows=10 width=1880)"
"  ->  Nested Loop  (cost=0.00..5430717.58 rows=2642 width=1880)"
"        ->  Index Scan Backward using
index_consolidateds_url_on_index_delta on consolidated_urls
(cost=0.00..5316175.98 rows=15242 width=1880)"
"              Filter: (index_delta IS NOT NULL)"
"        ->  Index Scan using
index_topical_urls_on_domain_id_and_consolidated_url_id on
topical_urls  (cost=0.00..7.50 rows=1 width=4)"
"              Index Cond: ((topical_urls.domain_id = 157) AND
(topical_urls.consolidated_url_id = consolidated_urls.id))"
"              Filter: (NOT topical_urls.hidden)"



The index_delta field is double precision and is indexed.

Any suggestions as to how to make this query run faster?