当运行下面的代码时：

drop table if exists demo;
drop table if exists demo_test;
drop table if exists demo_result;

create table demo as select md5(v::text) from generate_series(1, 1000000) v;
create index on demo (md5 text_pattern_ops);
analyze demo;

create table demo_test 
    as select left(md5(v::text), 5) || '%' as "patt" from generate_series(2000000, 2000010) v;

create table demo_result (row text);

load 'auto_explain';
set auto_explain.log_min_duration to 0;
set auto_explain.log_analyze to true;
set auto_explain.log_nested_statements to true;

do $$
declare
    row record;
pattern text;
begin
    for row in select patt from demo_test loop
        pattern = row.patt;  -- <--- CRUCIAL LINE
        insert into demo_result select * from demo where md5 like pattern;
    end loop;
end$$;

PostgreSQL生成以下查询计划：

2017-10-02 17:03:48 CEST [18038-23] app=psql barczynski@barczynski LOG:  duration: 0.021 ms  plan:
        Query Text: insert into demo_result select * from demo where md5 like pattern
        Insert on demo_result  (cost=0.42..8.45 rows=100 width=33) (actual time=0.021..0.021 rows=0 loops=1)
          ->  Index Only Scan using demo_md5_idx on demo  (cost=0.42..8.45 rows=100 width=33) (actual time=0.018..0.018 rows=1 loops=1)
                Index Cond: ((md5 ~>=~ '791cc'::text) AND (md5 ~<~ '791cd'::text))
                Filter: (md5 ~~ '791cc%'::text)
                Heap Fetches: 1

但在删除pattern变量，并在where条件下内联row.patt之后：

insert into demo_result select * from demo where md5 like row.patt;

PostgreSQL将参数视为绑定：

2017-10-02 17:03:02 CEST [17901-23] app=psql barczynski@barczynski LOG:  duration: 89.636 ms  plan:
        Query Text: insert into demo_result select * from demo where md5 like row.patt
        Insert on demo_result  (cost=0.00..20834.00 rows=5000 width=33) (actual time=89.636..89.636 rows=0 loops=1)
          ->  Seq Scan on demo  (cost=0.00..20834.00 rows=5000 width=33) (actual time=47.255..89.628 rows=1 loops=1)
                Filter: (md5 ~~ $4)
                Rows Removed by Filter: 999999

我知道后一个计划使用顺序扫描，因为PostgreSQL假设绑定参数以通配符开头。
我的问题是，为什么额外的赋值会打开和关闭绑定参数？