join has left join,right join,inner join;
join is inner join. This is the inner link.
the results of inner join and where are the same unless the results produced by left join,right join are different, that is, the query for the main table, such as
A left join B ON a.id = b.id
, in this case, the data of table An as the main table will be queried

SELECT * from posts,post_slugs the decal product of two tables
SELECT * from posts,post_slugs where post_slugs.post_id=posts.post_id is conditionally automatically optimized to the following statement
SELECT * from posts inner join post_slugs on post_slugs.post_id=posts.post_id
the difference between the two is that join must have conditional on, and the former can be unconditional.
the behavior of both before and after explain analyze analysis in postgresql is completely consistent, and the join time is slightly better than that of 0.05ms, which is estimated to be the time of multi-table optimization

explain analyze select * from t_xmxx_x a, t_shwzcmx b where a.f_xmid=b.f_xmid;
+----------------------------------------------------------------------------------------------------------------------------------------+
| QUERY PLAN                                                                                                                             |
|----------------------------------------------------------------------------------------------------------------------------------------|
| Nested Loop  (cost=0.29..8514.09 rows=27592 width=1412) (actual time=0.017..53.627 rows=27586 loops=1)                                 |
|   ->  Seq Scan on t_xmxx_x a  (cost=0.00..2578.49 rows=6149 width=955) (actual time=0.003..1.977 rows=6149 loops=1)                    |
|   ->  Index Scan using t_shwzcmx_index on t_shwzcmx b  (cost=0.29..0.89 rows=8 width=457) (actual time=0.002..0.005 rows=4 loops=6149) |
|         Index Cond: ((f_xmid)::text = (a.f_xmid)::text)                                                                                |
| Total runtime: 55.362 ms                                                                                                               |
+----------------------------------------------------------------------------------------------------------------------------------------+
EXPLAIN
Time: 0.061s


explain analyze select * from t_xmxx_x a join t_shwzcmx b on a.f_xmid=b.f_xmid;
+----------------------------------------------------------------------------------------------------------------------------------------+
| QUERY PLAN                                                                                                                             |
|----------------------------------------------------------------------------------------------------------------------------------------|
| Nested Loop  (cost=0.29..8514.09 rows=27592 width=1412) (actual time=0.020..53.727 rows=27586 loops=1)                                 |
|   ->  Seq Scan on t_xmxx_x a  (cost=0.00..2578.49 rows=6149 width=955) (actual time=0.002..1.879 rows=6149 loops=1)                    |
|   ->  Index Scan using t_shwzcmx_index on t_shwzcmx b  (cost=0.29..0.89 rows=8 width=457) (actual time=0.002..0.005 rows=4 loops=6149) |
|         Index Cond: ((f_xmid)::text = (a.f_xmid)::text)                                                                                |
| Total runtime: 55.319 ms                                                                                                               |
+----------------------------------------------------------------------------------------------------------------------------------------+
EXPLAIN
Time: 0.061s

data is too small and basically makes no difference
1 million. Try

SELECT * from posts,post_slugs where post_slugs.post_id=posts.post_id
SELECT * from posts join post_slugs on post_slugs.post_id=posts.post_id
these are just two different grammatical ways of writing, meaning and execution are the same.
the first SQL is not filtered through Cartesian product and then where. The following is
select * from (SELECT a.post_id as ida,b.post_id as idb from posts a postcards slugs b) c where c.idafuc.idb;