postgresql – Postgres 9.6并行XPath

我已经设置了Postgres 9.6并检查了一个大型的随机整数表,并行查询正在运行.
但是,对另一个表的 XML列的简单XPath查询始终是顺序的.两个XPath函数在Postgres中都标记为并行安全.我试图改变XPath成本,因此预期成本飙升,但它没有改变任何东西.
我错过了什么？

示例表DDL：
CREATE TABLE“test_table”(“xml”XML);

示例查询：
SELECT xpath(‘/ a’,“xml”)FROM“test_table”;

示例数据：
&LT a取代;< / a取代.
请注意,实际数据包含大小为10-1000kB的XML.

> select pg_size_pretty(pg_total_relation_size('test_table'));
28 MB

> explain (analyze,verbose,buffers) select xpath('/a',"xml") from test_table;
Seq Scan on public.test_table  (cost=0.00..64042.60 rows=2560 width=32) (actual time=1.420..4527.061 rows=2560 loops=1)
  Output: xpath('/a'::text,xml,'{}'::text[])
  Buffers: shared hit=10588
Planning time: 0.058 ms
Execution time: 4529.503 ms

解决方法

这里的相关点可能是“关系大小”和“总关系大小”之间的区别：

CREATE TABLE test_table AS
  SELECT ('<a>' || repeat('x',1000000) || '</a>')::xml AS "xml"
  FROM generate_series(1,2560);

SELECT
  pg_size_pretty(pg_relation_size('test_table')) AS relation_size,pg_size_pretty(pg_total_relation_size('test_table')) AS total_relation_size;

 relation_size | total_relation_size
---------------+---------------------
 136 kB        | 30 MB

像这样的大列值不存储在主关系中,而是被推送到其关联的TOAST table.此外部存储不计入pg_relation_size(),这是优化器在评估并行计划时与min_parallel_relation_size进行比较的结果：

SET parallel_setup_cost = 0;
SET parallel_tuple_cost = 0;
SET min_parallel_relation_size = '144kB';
EXPLAIN SELECT xpath('/a',"xml") FROM test_table;

                          QUERY PLAN
---------------------------------------------------------------
 Seq Scan on test_table  (cost=0.00..49.00 rows=2560 width=32)

SET min_parallel_relation_size = '136kB';
EXPLAIN SELECT xpath('/a',"xml") FROM test_table;

     QUERY PLAN
------------------------------------------------------------------------------
 Gather  (cost=0.00..38.46 rows=2560 width=32)
   Workers Planned: 1
   ->  Parallel Seq Scan on test_table  (cost=0.00..35.82 rows=1506 width=32)