YaeSakuraQ opened a new issue, #2480:
URL: https://github.com/apache/age/issues/2480
# A redundant length(p) = 2 check makes a fixed two-hop path about 11x slower
I found that adding `WHERE length(p) = 2` to a fixed two-hop path makes the
query about 11 times slower. The condition cannot remove any row because the
pattern already contains exactly two relationships. Both queries return the
same count.
- Apache AGE Version: 1.7.0
- PostgreSQL Version: 18.1
- Operating System: macOS 27.0 arm64
- Installation Method: official `apache/age` Docker image
- API/Driver: PostgreSQL wire protocol / Psycopg 3.3.4
### Steps to reproduce
1. Load AGE and create a fresh graph:
```sql
LOAD 'age';
SET search_path = ag_catalog, "$user", public;
SELECT ag_catalog.create_graph('fixed_path_length_perf');
```
2. Create 20,000 independent two-hop paths:
```sql
SELECT * FROM ag_catalog.cypher('fixed_path_length_perf', $$
UNWIND range(1, 20000) AS i
CREATE (:X {id: i})-[:P {id: i}]->(:Y {id: i})-[:Q {id: i}]->(:Z {id:
i})
RETURN count(*) AS created
$$) AS (created agtype);
```
3. Refresh the label statistics:
```sql
ANALYZE fixed_path_length_perf."X";
ANALYZE fixed_path_length_perf."Y";
ANALYZE fixed_path_length_perf."Z";
ANALYZE fixed_path_length_perf."P";
ANALYZE fixed_path_length_perf."Q";
```
4. Run query A:
```sql
SELECT * FROM ag_catalog.cypher('fixed_path_length_perf', $$
MATCH p=(:X)-[:P]->(:Y)-[:Q]->(:Z)
RETURN count(*) AS c
$$) AS (c agtype);
```
5. Run query B, which only adds a condition already guaranteed by the
pattern:
```sql
SELECT * FROM ag_catalog.cypher('fixed_path_length_perf', $$
MATCH p=(:X)-[:P]->(:Y)-[:Q]->(:Z)
WHERE length(p) = 2
RETURN count(*) AS c
$$) AS (c agtype);
```
6. Prefix both queries with `EXPLAIN (ANALYZE, BUFFERS)` to inspect their
plans.
### Expected behavior
Both queries should return `c = 20000`. Since the fixed pattern already
proves that every `p` has length 2, query B should not need to build and
measure the path for every matched row.
### Actual behavior
Both queries return `c = 20000`, but query B is consistently slower. Each
independent instance ran 12 alternating comparisons, and query B was slower in
all of them:
| Independent instance | Query A | Query B | Slowdown |
|---|---:|---:|---:|
| AGE 1.7.0, instance 1 | 16.94 ms | 195.88 ms | 11.57x |
| AGE 1.7.0, instance 2 | 17.18 ms | 192.34 ms | 11.19x |
The slow plan contains this additional per-row filter:
```text
Join Filter: (age_length(_agtype_build_path(...)) = '2'::agtype)
```
The fast plan has no corresponding path construction or length filter.
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