Re: ZStandard (with dictionaries) compression support for TOAST compression

Hi,

> Overall idea is great.
>
> I just want to mention LZ4 also have API to use dictionary. Its dictionary
> will be as simple as "virtually prepended" text (in contrast to complex
> ZStd dictionary format).
>
> I mean, it would be great if "dictionary" will be common property for
> different algorithms.
>
> On the other hand, zstd have "super fast" mode which is actually a bit
> faster than LZ4 and compresses a bit better. So may be support for
> different algos is not essential. (But then we need a way to change
> compression level to that "super fast" mode.)
>

zstd compression level and zstd dictionary size is configurable at
attribute level using ALTER TABLE. Default zstd level is 3 and dict
size is 4KB. For super fast mode level can be set to 1.

```
test=# alter table zstd alter column doc set compression zstd;
ALTER TABLE
test=# alter table zstd alter column doc set(zstd_cmp_level = 1);
ALTER TABLE
test=# select * from pg_attribute where attrelid = 'zstd'::regclass
and attname = 'doc';
attrelid | attname | atttypid | attlen | attnum | atttypmod |
attndims | attbyval | attalign | attstorage | attcompre
ssion | attnotnull | atthasdef | atthasmissing | attidentity |
attgenerated | attisdropped | attislocal | attinhcount
| attcollation | attstattarget | attacl | attoptions
| attfdwoptions | attmissingval
----------+---------+----------+--------+--------+-----------+----------+----------+----------+------------+----------
------+------------+-----------+---------------+-------------+--------------+--------------+------------+-------------
+--------------+---------------+--------+----------------------------------+---------------+---------------
16389 | doc | 3802 | -1 | 1 | -1 |
0 | f | i | x | z
| f | f | f | |
| f | t | 0
| 0 | | |
{zstd_dictid=1,zstd_cmp_level=1} | |
(1 row)
```