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=#