RE: Initial COPY of Logical Replication is too slow

[Hayato Kuroda (Fujitsu)]

> I ran the performance testing independently for the 0001 patch. Overall
> performance looked
> very nice, new function spent O(1) time based on the total number of tables.
> It seems good enough.

...and I tested 0002 as well with the same settings, and the trend was the same 
as 0001.

Both 0001 and 0002 were applied and below SQL was run, which was same was what
tablesync worker would try:

```
SELECT DISTINCT
  (CASE WHEN (array_length(gpt.attrs, 1) = c.relnatts)
   THEN NULL ELSE gpt.attrs END)
  FROM pg_get_publication_tables(ARRAY['pub'], 16535) gpt,
  pg_class c
 WHERE c.oid = gpt.relid;
```

And below is the result. Each cell shows the execution time of the SQL. HEAD
column is the case when [1] was done. 0001 column is the case for [2].
Looks like the SQL used by 0002 looks slightly faster, which is same as the
expectation. JOIN was removed once.

Total tables    HEAD [ms]       0001 [ms]       0001 + 0002 [ms]
50              5.77            4.19            3.74
500             15.75           4.28            3.76
5000            120.39          4.22            3.79
50000           1741.89         4.60            4.11
500000          73287.16        4.95            4.38

Attached graph visualized the table.

[1]:
```
SELECT DISTINCT
  (CASE WHEN (array_length(gpt.attrs, 1) = c.relnatts)
   THEN NULL ELSE gpt.attrs END)
  FROM pg_publication p,
  LATERAL pg_get_publication_tables(p.pubname) gpt,
  pg_class c
 WHERE gpt.relid = 17885 AND c.oid = gpt.relid
   AND p.pubname IN ( 'pub' );
```

[2]:
```
SELECT DISTINCT
  (CASE WHEN (array_length(gpt.attrs, 1) = c.relnatts)
   THEN NULL ELSE gpt.attrs END)
  FROM pg_publication p,
  LATERAL pg_get_publication_tables(p.pubname, 16535) gpt,
  pg_class c
 WHERE c.oid = gpt.relid
   AND p.pubname IN ( 'pub' );
```

Best regards,
Hayato Kuroda
FUJITSU LIMITED