PL/pgSQL Query Plan Worse Inside Function Than Outside - sql

Question

I have a function that is running too slow. I've isolated which piece of the function is slow.. a small SELECT statement:
SELECT image_group_id
FROM programs.image_family fam
JOIN programs.provider_file pf
ON (fam.provider_data_id = pf.provider_data_id
AND fam.family_id = $1 AND pf.image_group_id IS NOT NULL)
LIMIT 1
When I run the function this piece of SQL generates the following query plan:
Query Text: SELECT image_group_id FROM programs.image_family fam JOIN programs.provider_file pf ON (fam.provider_data_id = pf.provider_data_id AND fam.family_id = $1 AND pf.image_group_id IS NOT NULL) LIMIT 1
Limit (cost=0.56..6.75 rows=1 width=6) (actual time=3471.004..3471.004 rows=0 loops=1)
-> Nested Loop (cost=0.56..594054.42 rows=96017 width=6) (actual time=3471.002..3471.002 rows=0 loops=1)
-> Seq Scan on image_family fam (cost=0.00..391880.08 rows=96023 width=6) (actual time=3471.001..3471.001 rows=0 loops=1)
Filter: ((family_id)::numeric = '8419853'::numeric)
Rows Removed by Filter: 19204671
-> Index Scan using "IX_DBO_PROVIDER_FILE_1" on provider_file pf (cost=0.56..2.11 rows=1 width=12) (never executed)
Index Cond: (provider_data_id = fam.provider_data_id)
Filter: (image_group_id IS NOT NULL)
When I run the selected query in a query tool (outside of the function) the query plan looks like this:
Limit (cost=1.12..3.81 rows=1 width=6) (actual time=0.043..0.043 rows=1 loops=1)
Output: pf.image_group_id
Buffers: shared hit=11
-> Nested Loop (cost=1.12..14.55 rows=5 width=6) (actual time=0.041..0.041 rows=1 loops=1)
Output: pf.image_group_id
Inner Unique: true
Buffers: shared hit=11
-> Index Only Scan using image_family_family_id_provider_data_id_idx on programs.image_family fam (cost=0.56..1.65 rows=5 width=6) (actual time=0.024..0.024 rows=1 loops=1)
Output: fam.family_id, fam.provider_data_id
Index Cond: (fam.family_id = 8419853)
Heap Fetches: 2
Buffers: shared hit=6
-> Index Scan using "IX_DBO_PROVIDER_FILE_1" on programs.provider_file pf (cost=0.56..2.58 rows=1 width=12) (actual time=0.013..0.013 rows=1 loops=1)
Output: pf.provider_data_id, pf.provider_file_path, pf.posted_dt, pf.file_repository_id, pf.restricted_size, pf.image_group_id, pf.is_master, pf.is_biggest
Index Cond: (pf.provider_data_id = fam.provider_data_id)
Filter: (pf.image_group_id IS NOT NULL)
Buffers: shared hit=5
Planning time: 0.809 ms
Execution time: 0.100 ms
If I disable sequence scans in the function I can get a similar query plan:
Query Text: SELECT image_group_id FROM programs.image_family fam JOIN programs.provider_file pf ON (fam.provider_data_id = pf.provider_data_id AND fam.family_id = $1 AND pf.image_group_id IS NOT NULL) LIMIT 1
Limit (cost=1.12..8.00 rows=1 width=6) (actual time=3855.722..3855.722 rows=0 loops=1)
-> Nested Loop (cost=1.12..660217.34 rows=96017 width=6) (actual time=3855.721..3855.721 rows=0 loops=1)
-> Index Only Scan using image_family_family_id_provider_data_id_idx on image_family fam (cost=0.56..458043.00 rows=96023 width=6) (actual time=3855.720..3855.720 rows=0 loops=1)
Filter: ((family_id)::numeric = '8419853'::numeric)
Rows Removed by Filter: 19204671
Heap Fetches: 368
-> Index Scan using "IX_DBO_PROVIDER_FILE_1" on provider_file pf (cost=0.56..2.11 rows=1 width=12) (never executed)
Index Cond: (provider_data_id = fam.provider_data_id)
Filter: (image_group_id IS NOT NULL)
The query plans are different where the Filter functions are for the Index Only Scan. The function has more Heap Fetches and seems to treat the argument as a string casted to a numeric.
Things I've tried:
Increasing statistics (and running vacuum/analyze)
Calling the problematic piece of SQL in another function with language SQL
Add another index (the one that its using now to perform an INDEX ONLY scan)
Create a CTE for the image_family table (this did help performance but would still do a sequence scan on the image_family instead of using the index so still, too slow)
Change from executing raw SQL to using an EXECUTE ... INTO .. USING in the function.
Makeup of the two tables:
image_family:
provider_data_id: numeric(16)
family_id: int4
(rest omitted for brevity)
unique index on provider_data_id
index on family_id
I recently added a unique index on (family_id, provider_data_id) as well
Approximately 20 million rows here. Families have many provider_data_ids but not all provider_data_ids are part of families and thus aren't all in this table.
provider_file:
provider_data_id numeric(16)
image_group_id numeric(16)
(rest omitted for brevity)
unique index on provider_data_id
Approximately 32 million rows in this table. Most rows (> 95%) have a Non-Null image_group_id.
Postgres Version 10
How can I get the query performance to match whether I call it from a function or as raw SQL in a query tool?

Answer 1

The problem is exhibited in this line:
Filter: ((family_id)::numeric = '8419853'::numeric)
The index on family_id cannot be used because family_id is compared to a numeric value. This requires a cast to numeric, and there is no index on family_id::numeric.
Even though integer and numeric both are types representing numbers, their internal representation is quite different, and so the indexes are incompatible. In other words, the cast to numeric is like a function for PostgreSQL, and since it has no index on that functional expression, it has to resort to a scan of the whole table (or index).
The solution is simple, however: use an integer instead of a numeric parameter for the query. If in doubt, use a cast like
fam.family_id = $1::integer