PostgreSQL DBA(57) - Could not choose a best candidate operator

本节内容来源于客户现场反馈的一个问题.
Question
创建了integer到text的cast后,为何执行字符串连接”||”操作报错?

testdb=# drop table if exists t_cast ;
DROP TABLE
testdb=# create table t_cast (id int);
CREATE TABLE
testdb=# insert into t_cast values(1),(2),(3);
INSERT 0 3
testdb=# create cast(integer as text) with inout as implicit;
CREATE CAST
testdb=# select id||'X' from t_cast;
psql: ERROR:  operator is not unique: integer || unkNown
LINE 1: select id||'X' from t_cast;
                 ^
HINT:  Could not choose a best candidate operator. You might need to add explicit type casts.

Answer
Postgresql处理操作符时分为以下两个步骤:
1.从系统目录pg_operator中找出匹配该操作符和操作数的候选函数(数据行);
2.根据隐式转换规则选择合适的operator.

原生PG
通过分析和跟踪代码,第一步,从pg_operator中选出的后续函数OID为374/2780

testdb=# select oid,oprname,oprleft::regtype,oprright::regtype 
testdb-# from pg_operator 
testdb-# where oid in (374,2780);
 oid  | oprname |   oprleft   | oprright 
------+---------+-------------+----------
  374 | ||      | anyelement  | anyarray
 2780 | ||      | anynonarray | text
(2 rows)

而’X’可视为text,最终Postgresql会选择OID = 2780的operator,因此不会出错.

创建integer -> text转换
而创建了integer -> text的cast后,从pg_operator中选出的后续函数OID为654/2779/374/2780

testdb=# select oid,oprname,oprleft::regtype,oprright::regtype 
testdb-# from pg_operator 
testdb-# where oid in (654,2779,374,2780);
 oid  | oprname |   oprleft   |  oprright   
------+---------+-------------+-------------
  374 | ||      | anyelement  | anyarray
  654 | ||      | text        | text
 2779 | ||      | text        | anynonarray
 2780 | ||      | anynonarray | text
(4 rows)

因为integer可以转换为text,Postgresql无法在2779和2780之间无法做出选择,因此出现错误:Could not choose a best candidate operator.

参考资料
Postgresql 源码解读（209）- 隐式类型转换(func_select_candidate)
Postgresql 源码解读（210）- 隐式类型转换(func_match_argtypes)