在PostgreSQL数据库中使用$$作为字符串的引用符号

参考自： https://www.postgresql.org/docs/current/static/sql-syntax-lexical.html
中的4.1.2.4. Dollar-quoted String Constants部分。


大概意思是：$highgolei$t$highgolei$中的 highgolei是Tag，这Tag可以不要，于是就成了dollar dollar t dollar dollar ，这个dollar dollar t dollar dollar就等于't'
这个Tag啥用？没自己研究。。。

postgres=# select count(*) from pg_class where relkind='t';
 count 
-------
    19
(1 row)


postgres=# select count(*) from pg_class where relkind=$$t$$;
 count 
-------
    19
(1 row)


postgres=# select count(*) from pg_class where relkind=$SomeTag$Dianne's horse$SomeTag$;
 count 
-------
     0
(1 row)


postgres=# select count(*) from pg_class where relkind=$SomeTag$t$SomeTag$;
 count 
-------
    19
(1 row)
postgres=# select count(*) from pg_class where relkind=$highgo$t$highgo$;
 count 
-------
    19
(1 row)


postgres=# select count(*) from pg_class where relkind=$highgolei$t$highgolei$;
 count 
-------
    19
(1 row)


postgres=# select count(*) from pg_class where relkind=$highgo-lei$t$highgo-lei$;
ERROR:  syntax error at or near "$"
LINE 1: select count(*) from pg_class where relkind=$highgo-lei$t$hi...
                                                    ^
postgres=#