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A btree index scan on agtype that starts at a `>=` bound skips the keys equal to it (`n.id >= 995` returns 996 to 1000)

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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
48/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Attiva
Stack tecnologico
c, postgresql
Ambito
databases

Direzione di ricerca

Reproduce the missing-bound-row cases with the plain agtype table and the graph index setup. Start by inspecting the agtype_ops_btree strategy mappings, agtype_btree_cmp, and agtype_ge, then trace how the forward index scan is positioned at a >= bound. Done means indexed queries return the same rows as sequential scans, including equal bounds and closed ranges.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Describe the bug

A btree index scan on an agtype key that starts at a >= bound skips the keys equal to the bound. MATCH (n:P) WHERE n.id >= 995 RETURN n.id through the documented property expression index returns 996 to 1000; without the index it returns 995 to 1000. A closed range n.id >= 500 AND n.id <= 510 returns 10 rows instead of 11, and a >= 5 AND a <= 5 returns none while a = 5 returns one.

It is not specific to Cypher or to expression indexes: a plain agtype column with a default btree index does the same, for integer, float and string values, and on a 10-row index that fits on one leaf page. Through the same index, >, <, <= and = return the right rows, and so does >= when the scan does not start at it (a DESC index, or ORDER BY a DESC). A >= bound with no equal key (a >= 4.5) is also right. A float8 column with the same data and queries on the same server is right.

What I checked: agtype_ops_btree maps strategies 1 to 5 to <, <=, =, >, >= on (agtype, agtype); agtype_btree_cmp returns 0 for a stored value against the equal constant, in both argument orders; and agtype_ge returns true for that pair. So the functions agree that the values are equal, and the row is lost when a forward scan is positioned at the >= bound. I have not found the line responsible.

We found this in a benchmark: after the write phase, a read-back MATCH (q:Person) WHERE q.id >= $f RETURN q.id, ... on AGE returned one person fewer than eight other graph engines. The planner chose the index scan itself there (2,000 persons), so no planner settings are involved in the application.

How are you accessing AGE (Command line, driver, etc.)?

psql, and psycopg 3 in the application (same results).

What data setup do we need to do?

CREATE EXTENSION IF NOT EXISTS age;
LOAD 'age';
SET search_path = ag_catalog, "$user", public;
SELECT create_graph('rng');
SELECT * FROM cypher('rng', $$ UNWIND range(1, 1000) AS i CREATE (:P {id: i}) $$) AS (v agtype);
CREATE INDEX p_id ON rng."P" USING btree (agtype_access_operator(properties, '"id"'::agtype));
ANALYZE rng."P";

What is the necessary configuration info needed?

None. On a table this small the planner may prefer a sequential scan, so the repro sets enable_seqscan = off and enable_bitmapscan = off to make it take the index.

What is the command that caused the error?

SET enable_seqscan = off;
SET enable_bitmapscan = off;
SELECT string_agg(v::text, ',') FROM cypher('rng', $$ MATCH (n:P) WHERE n.id >= 995 RETURN n.id $$) AS (v agtype);
-- 996,997,998,999,1000
SELECT count(*) FROM cypher('rng', $$ MATCH (n:P) WHERE n.id >= 500 AND n.id <= 510 RETURN n $$) AS (v agtype);
-- 10

The same without a graph:

CREATE TABLE t (a agtype);
INSERT INTO t SELECT i::text::agtype FROM generate_series(1, 10) i;
CREATE INDEX ON t (a);
SET enable_seqscan = off;
SET enable_bitmapscan = off;
SELECT string_agg(a::text, ',') FROM t WHERE a >= '5'::agtype;        -- 6,7,8,9,10
SELECT count(*) FROM t WHERE a >= '5'::agtype AND a <= '5'::agtype;   -- 0
SELECT count(*) FROM t WHERE a = '5'::agtype;                         -- 1
SELECT string_agg(a::text, ',') FROM t WHERE a > '4'::agtype;         -- 5,6,7,8,9,10

Expected behavior

The index returns the rows the predicate selects without it: 995 to 1000; 11 rows; 5 to 10; 1; 1; 5 to 10.

Environment (please complete the following information):

The same results on every image tried:

image PostgreSQL AGE
apache/age:release_PG18_1.8.0 18.6 1.8.0
apache/age:dev_snapshot_master 18.6 master
apache/age:dev_snapshot_PG18 18.4 PG18 branch
apache/age:release_PG17_1.7.0 17.11 1.7.0
apache/age:release_PG16_1.6.0 16.10 1.6.0
pgvector/pgvector pg18 + PGDG postgresql-18-age 18.6 1.8.0

Additional context

Results through the index on the 1,000-vertex graph above, and on a 1,000-row plain agtype column:

predicate through the index without the index
>= 995 996 to 1000 995 to 1000
> 994 995 to 1000 995 to 1000
<= 5 1 to 5 1 to 5
>= 500 AND <= 510 10 rows 11 rows
= 995 1 row 1 row
>= 995, DESC index 995 to 1000 995 to 1000
Lingua principale
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