amcheck complains about a btree index build by pg_bulkload with merge

[harukat]

As far as we have been able to ascertain, btree indexes built by pg_bulkload with merging
are usually found to have some problems when examined with the functions of amcheck.
However, it does not seem to cause any errors when various searches are performed using such indexes.

Is this expected behavior and an unavoidable limitation due to pg_bulkload performing data loading
in an unorthodox manner?

[harukat]

Here is the reproduce script and a result.

[postgres@host1]$ cat reproduce.sh
dropdb --if-exists reprodb
createdb reprodb
psql reprodb <<'EOS'
CREATE EXTENSION pg_bulkload;
CREATE EXTENSION amcheck;
\dx
SELECT version();
CREATE TABLE t_dup (c int);
CREATE INDEX idx_t_dup ON t_dup (c);
INSERT INTO t_dup SELECT generate_series(1, 10);
INSERT INTO t_dup SELECT generate_series(1, 10);
INSERT INTO t_dup SELECT generate_series(1, 10);
\copy t_dup TO /tmp/t_dup.csv WITH (FORMAT CSV)
TRUNCATE t_dup;

EOS

cat > /tmp/repro.ctl <<EOS
OUTPUT    = t_dup
INPUT     = /tmp/t_dup.csv
TYPE      = CSV
DELIMITER = ","
SKIP      = 0
LOGFILE   = /tmp/repro.log
PARSE_BADFILE = /tmp/repro_bad.log
WRITER = DIRECT

EOS

pg_bulkload --elevel=debug -d reprodb /tmp/repro.ctl
pg_bulkload --elevel=debug -d reprodb /tmp/repro.ctl

psql -e -d reprodb <<EOS
SELECT * FROM bt_index_check('idx_t_dup');

EOS

psql -e -d reprodb <<EOS
SELECT * FROM bt_index_parent_check('idx_t_dup');

EOS

[postgres@host1]$ sh reproduce.sh
CREATE EXTENSION
CREATE EXTENSION
                                     List of installed extensions
    Name     | Version |   Schema   |                           Description     
-------------+---------+------------+-----------------------------------------------------------------
 amcheck     | 1.4     | public     | functions for verifying relation integrity
 pg_bulkload | 3.1.22  | public     | pg_bulkload is a high speed data loading utility for PostgreSQL
 plpgsql     | 1.0     | pg_catalog | PL/pgSQL procedural language
(3 rows)

                                                 version                        
---------------------------------------------------------------------------------------------------------
 PostgreSQL 17.6 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-22), 64-bit
(1 row)

CREATE TABLE
CREATE INDEX
INSERT 0 10
INSERT 0 10
INSERT 0 10
COPY 30
TRUNCATE TABLE
NOTICE: BULK LOAD START
NOTICE: BULK LOAD END
        0 Rows skipped.
        30 Rows successfully loaded.
        0 Rows not loaded due to parse errors.
        0 Rows not loaded due to duplicate errors.
        0 Rows replaced with new rows.
NOTICE: BULK LOAD START
NOTICE: BULK LOAD END
        0 Rows skipped.
        30 Rows successfully loaded.
        0 Rows not loaded due to parse errors.
        0 Rows not loaded due to duplicate errors.
        0 Rows replaced with new rows.

SELECT * FROM bt_index_check('idx_t_dup');
ERROR:  item order invariant violated for index "idx_t_dup"
DETAIL:  Lower index tid=(1,3) (points to heap tid=(1,21)) higher index tid=(1,4) (points to heap tid=(0,1)) page lsn=0/1E35BA28.

SELECT * FROM bt_index_parent_check('idx_t_dup');
ERROR:  item order invariant violated for index "idx_t_dup"
DETAIL:  Lower index tid=(1,3) (points to heap tid=(1,21)) higher index tid=(1,4) (points to heap tid=(0,1)) page lsn=0/1E35BA28.