PG17: SELECT distinct のEXPLAINのnrowsが実行のたびに変動する

[0-kaz]

SELECT distinctを含むクエリのEXPLAIN出力中にあるnrowsの行数が実行のたびに変わってしまう。
再現性が低い現象であったがVACUUM ANALYZEを直前に実行すると100%再現することを確認。

実行計画：

1回目

Sort
Output: customer.c_nation, lineorder.lo_orderpriority
Sort Key: customer.c_nation, lineorder.lo_orderpriority
-> HashAggregate
Output: customer.c_nation, lineorder.lo_orderpriority
Group Key: customer.c_nation, lineorder.lo_orderpriority
-> Custom Scan (GpuPreAgg) on public.lineorder
Output: customer.c_nation, lineorder.lo_orderpriority
GPU Projection: customer.c_nation, lineorder.lo_orderpriority
GPU Scan Quals: (lineorder.lo_shipmode = 'AIR'::bpchar) [rows: 6001260 -> 874784]
GPU Join Quals [1]: (lineorder.lo_custkey = customer.c_custkey) ... [nrows: 874784 -> 174724]

2回目

Sort
Output: customer.c_nation, lineorder.lo_orderpriority
Sort Key: customer.c_nation, lineorder.lo_orderpriority
-> HashAggregate
Output: customer.c_nation, lineorder.lo_orderpriority
Group Key: customer.c_nation, lineorder.lo_orderpriority
-> Custom Scan (GpuPreAgg) on public.lineorder
Output: customer.c_nation, lineorder.lo_orderpriority
GPU Projection: customer.c_nation, lineorder.lo_orderpriority
GPU Scan Quals: (lineorder.lo_shipmode = 'AIR'::bpchar) [rows: 6001260 -> 845177]
GPU Join Quals [1]: (lineorder.lo_custkey = customer.c_custkey) ... [nrows: 845177 -> 174724]

データ投入

DROP TABLE IF EXISTS customer;
CREATE TABLE customer (
    c_custkey   integer NOT NULL,
    c_name      character varying(25),
    c_address   character varying(25),
    c_city      character(10),
    c_nation    character(15),
    c_region    character(12),
    c_phone     character(15),
    c_mktsegment character(10)
);

DROP TABLE IF EXISTS lineorder;
CREATE TABLE lineorder (
    lo_orderkey     bigint,
    lo_linenumber   integer,
    lo_custkey      integer,
    lo_partkey      integer,
    lo_suppkey      integer,
    lo_orderdate    integer,
    lo_orderpriority character(15),
    lo_shippriority character(1),
    lo_quantity     numeric,
    lo_extendedprice numeric,
    lo_ordertotalprice numeric,
    lo_discount     numeric,
    lo_revenue      numeric,
    lo_supplycost   numeric,
    lo_tax          numeric,
    lo_commit_date  character(8),
    lo_shipmode     character(10)
);

\copy customer  FROM PROGRAM './ssbm/dbgen-ssbm -s 1 -X -Tc' DELIMITER '|'
\copy lineorder FROM PROGRAM './ssbm/dbgen-ssbm -s 1 -X -Tl' DELIMITER '|'

再現クエリ(VACUUM ANALYZEを直前に実行すると100%再現）

VACUUM ANALYZE;
explain (verbose, costs off)
SELECT * FROM reference EXCEPT select distinct c_nation, lo_orderpriority
  from lineorder, customer
 where lo_custkey = c_custkey
   and c_region = 'AMERICA'
   and lo_shipmode = 'AIR'
 order by c_nation, lo_orderpriority;

[kaigai]

これ、最新版のPG-Stromだと再現しなくなってますね・・・。
（今日、何回かregression testを流しても、ココで引っかからなかった）