Very bad plan when using VIEW and IN (SELECT...*)

Ref these two queries against a view:

-- QUERY 1, executes < 0.5 secs
SELECT *
FROM mdx_core.vw_provider AS p
WHERE provider_id IN (13083101)

-- QUERY 2, executes > 13.5 secs
SELECT *
FROM mdx_core.vw_provider AS p
WHERE provider_id IN (SELECT 13083101)

I am using the simple IN (SELECT n) in QUERY 2 to simplify the problem. I
noticed the oddity of the behaviour when I used a proper "IN (SELECT myId
FROM myTable)" but the planner shows the same behaviour even if not
selecting from a table - just the SELECT keyword is enough.

Plans are below. The view has an internal UNION.
Any explanation as to why this happens?

The actualt view is listed at the very bottom, if relevant.

Carlo

QUERY 1 PLAN
"Unique (cost=25.48..25.69 rows=2 width=417) (actual time=0.180..0.190
rows=2 loops=1)"
" -> Sort (cost=25.48..25.48 rows=2 width=417) (actual time=0.179..0.180
rows=2 loops=1)"
" Sort Key: "*SELECT* 1".provider_id, (NULL::integer), "*SELECT*
1".master_id, "*SELECT* 1".client_ids, "*SELECT* 1".upin, "*SELECT*
1".medical_education_number, "*SELECT* 1".abmsuid, "*SELECT* 1".npi,
"*SELECT* 1".npi_status_code, "*SELECT* 1".cc_id, "*SELECT* 1".aoa_id,
"*SELECT* 1".last_name, "*SELECT* 1".first_name, "*SELECT* 1".middle_name,
"*SELECT* 1".suffix, "*SELECT* 1".display_name, "*SELECT* 1".display_title,
"*SELECT* 1".nickname, "*SELECT* 1".familiar_name, "*SELECT* 1".pubmed_name,
"*SELECT* 1".master_name, "*SELECT* 1".display_name_orig, (NULL::text),
"*SELECT* 1".gender, "*SELECT* 1".birth_year, "*SELECT* 1".birth_month,
"*SELECT* 1".birth_day, "*SELECT* 1".clinical_interest, "*SELECT*
1".research_interest, "*SELECT* 1".summary, "*SELECT* 1".comments, "*SELECT*
1".degree_types, "*SELECT* 1".provider_type_ids, "*SELECT*
1".provider_status_code, "*SELECT* 1".provider_status_year, "*SELECT*
1".created, "*SELECT* 1".unique_flag, "*SELECT* 1".is_locked, "*SELECT*
1".provider_standing_code, "*SELECT* 1".impt_source_date, "*SELECT*
1".input_resource_id, "*SELECT* 1".input_source_ids"
" Sort Method: quicksort Memory: 27kB"
" -> Append (cost=0.00..25.47 rows=2 width=417) (actual
time=0.078..0.143 rows=2 loops=1)"
" -> Subquery Scan "*SELECT* 1" (cost=0.00..8.59 rows=1
width=408) (actual time=0.078..0.079 rows=1 loops=1)"
" -> Index Scan using provider_provider_id_idx on
provider p (cost=0.00..8.58 rows=1 width=408) (actual time=0.076..0.077
rows=1 loops=1)"
" Index Cond: (provider_id = 13083101)"
" -> Subquery Scan "*SELECT* 2" (cost=0.00..16.87 rows=1
width=417) (actual time=0.061..0.062 rows=1 loops=1)"
" -> Nested Loop (cost=0.00..16.86 rows=1 width=417)
(actual time=0.055..0.056 rows=1 loops=1)"
" -> Index Scan using
provider_name_pid_rec_stat_idx on provider_alias pa (cost=0.00..8.27 rows=1
width=32) (actual time=0.047..0.047 rows=1 loops=1)"
" Index Cond: (provider_id = 13083101)"
" -> Index Scan using provider_provider_id_idx on
provider p (cost=0.00..8.58 rows=1 width=389) (actual time=0.005..0.006
rows=1 loops=1)"
" Index Cond: (p.provider_id = 13083101)"
"Total runtime: 0.371 ms"

QUERY 2 PLAN
"Merge IN Join (cost=2421241.80..3142039.99 rows=30011 width=2032) (actual
time=13778.400..13778.411 rows=2 loops=1)"
" Merge Cond: ("*SELECT* 1".provider_id = (13083101))"
" -> Unique (cost=2421241.77..3066486.33 rows=6002275 width=417) (actual
time=13778.119..13778.372 rows=110 loops=1)"
" -> Sort (cost=2421241.77..2436247.46 rows=6002275 width=417)
(actual time=13778.118..13778.163 rows=110 loops=1)"
" Sort Key: "*SELECT* 1".provider_id, (NULL::integer),
"*SELECT* 1".master_id, "*SELECT* 1".client_ids, "*SELECT* 1".upin,
"*SELECT* 1".medical_education_number, "*SELECT* 1".abmsuid, "*SELECT*
1".npi, "*SELECT* 1".npi_status_code, "*SELECT* 1".cc_id, "*SELECT*
1".aoa_id, "*SELECT* 1".last_name, "*SELECT* 1".first_name, "*SELECT*
1".middle_name, "*SELECT* 1".suffix, "*SELECT* 1".display_name, "*SELECT*
1".display_title, "*SELECT* 1".nickname, "*SELECT* 1".familiar_name,
"*SELECT* 1".pubmed_name, "*SELECT* 1".master_name, "*SELECT*
1".display_name_orig, (NULL::text), "*SELECT* 1".gender, "*SELECT*
1".birth_year, "*SELECT* 1".birth_month, "*SELECT* 1".birth_day, "*SELECT*
1".clinical_interest, "*SELECT* 1".research_interest, "*SELECT* 1".summary,
"*SELECT* 1".comments, "*SELECT* 1".degree_types, "*SELECT*
1".provider_type_ids, "*SELECT* 1".provider_status_code, "*SELECT*
1".provider_status_year, "*SELECT* 1".created, "*SELECT* 1".unique_flag,
"*SELECT* 1".is_locked, "*SELECT* 1".provider_standing_code, "*SELECT*
1".impt_source_date, "*SELECT* 1".input_resource_id, "*SELECT*
1".input_source_ids"
" Sort Method: external merge Disk: 423352kB"
" -> Append (cost=0.00..596598.30 rows=6002275 width=417)
(actual time=0.039..7879.715 rows=1312637 loops=1)"
" -> Subquery Scan "*SELECT* 1" (cost=0.00..543238.96
rows=5994998 width=408) (actual time=0.039..7473.664 rows=1305360 loops=1)"
" -> Seq Scan on provider p (cost=0.00..483288.98
rows=5994998 width=408) (actual time=0.037..6215.112 rows=1305360 loops=1)"
" -> Subquery Scan "*SELECT* 2" (cost=0.00..53359.34
rows=7277 width=417) (actual time=0.049..186.643 rows=7277 loops=1)"
" -> Nested Loop (cost=0.00..53286.57 rows=7277
width=417) (actual time=0.043..176.134 rows=7277 loops=1)"
" -> Seq Scan on provider_alias pa
(cost=0.00..157.77 rows=7277 width=32) (actual time=0.018..3.134 rows=7277
loops=1)"
" -> Index Scan using
provider_provider_id_idx on provider p (cost=0.00..7.29 rows=1 width=389)
(actual time=0.021..0.021 rows=1 loops=7277)"
" Index Cond: (p.provider_id =
pa.provider_id)"
" -> Sort (cost=0.03..0.04 rows=1 width=4) (actual time=0.014..0.014
rows=1 loops=1)"
" Sort Key: (13083101)"
" Sort Method: quicksort Memory: 25kB"
" -> Result (cost=0.00..0.01 rows=1 width=0) (actual
time=0.001..0.001 rows=1 loops=1)"
"Total runtime: 13959.905 ms"

REATE OR REPLACE VIEW mdx_core.vw_provider AS
SELECT
p.provider_id,
NULL AS provider_alias_id,
p.master_id,
p.client_ids,
p.upin,
p.medical_education_number,
p.abmsuid,
p.npi,
p.npi_status_code,
p.cc_id,
p.aoa_id,
p.last_name,
p.first_name,
p.middle_name,
p.suffix,
p.display_name,
p.display_title,
p.nickname,
p.familiar_name,
p.pubmed_name,
p.master_name,
p.display_name_orig,
NULL::text AS is_primary,
p.gender,
p.birth_year,
p.birth_month,
p.birth_day,
p.clinical_interest,
p.research_interest,
p.summary,
p.comments,
p.degree_types,
p.provider_type_ids,
p.provider_status_code,
p.provider_status_year,
p.created,
p.unique_flag,
p.is_locked,
p.provider_standing_code,
p.impt_source_date,
p.input_resource_id,
p.input_source_ids
FROM mdx_core.provider AS p

UNION SELECT
p.provider_id,
pa.provider_alias_id,
p.master_id,
p.client_ids,
p.upin,
p.medical_education_number,
p.abmsuid,
p.npi,
p.npi_status_code,
p.cc_id,
p.aoa_id,
pa.last_name,
pa.first_name,
pa.middle_name,
pa.suffix,
p.display_name,
p.display_title,
p.nickname,
p.familiar_name,
p.pubmed_name,
p.master_name,
p.display_name_orig,
pa.is_primary,
p.gender,
p.birth_year,
p.birth_month,
p.birth_day,
p.clinical_interest,
p.research_interest,
p.summary,
p.comments,
p.degree_types,
p.provider_type_ids,
p.provider_status_code,
p.provider_status_year,
p.created,
p.unique_flag,
p.is_locked,
p.provider_standing_code,
p.impt_source_date,
p.input_resource_id,
p.input_source_ids
FROM mdx_core.provider_alias AS pa
JOIN mdx_core.provider AS p USING (provider_id);