问题描述
在 Postgresql 中,当我运行时:
EXPLAIN (FORMAT JSON) SELECT * FROM ACCOUNT WHERE TYPE > 'CHK'
我明白了:
[
{
"Plan": {
"Node Type": "Seq Scan","Parallel Aware": false,"Relation Name": "account","Alias": "account","Startup Cost": 0.00,"Total Cost": 13.25,"Plan Rows": 87,"Plan Width": 276,"Filter": "((type)::text > 'CHK'::text)"
}
}
]
我如何从那里获得“总成本”?我试过了:
SELECT P -> 'Total Cost' FROM (
EXPLAIN (FORMAT JSON)
SELECT * FROM ACCOUNT WHERE TYPE > 'CHK'
) X (P)
但我收到错误:
ERROR: Syntax error at or near "JSON"
Position: 51
sqlState: 42601
解决方法
explain
总是需要放在语句之前,不能放在语句中间。
你可以把它包装成一个函数来做你想做的事:
create or replace function get_plan_cost(p_sql text)
returns text
as
$$
declare
l_result jsonb;
begin
execute 'explain (format json) ' || p_sql
into l_result;
return l_result #>> '{0,Plan,"Total Cost"}';
end;
$$
language plpgsql;
select get_plan_cost($$ SELECT * FROM ACCOUNT WHERE TYPE > 'CHK' $$);
如果不能使用函数,可以使用带有 RAISE 语句的匿名块:
set client_min_messages=notice;
do
$$
declare
l_sql text := $$ SELECT * FROM ACCOUNT WHERE TYPE > 'CHK' $$;
l_result jsonb;
begin
execute 'explain (format json) ' || l_sql
into l_result;
raise notice 'Cost: %',l_result #>> '{0,"Total Cost"}';
end;
$$
;
但这与使用相比似乎过于复杂:
explain (format text)
SELECT * FROM ACCOUNT WHERE TYPE > 'CHK';
然后只看第一行。