On 5/27/21 4:15 AM, Andrew Dunstan wrote:
>
> On 5/26/21 5:29 PM, Bruce Momjian wrote:
>> On Tue, May 25, 2021 at 01:55:13PM +0300, Aleksander Alekseev wrote:
>>> Hi hackers,
>>>
>>> Back in 2016 while being at PostgresPro I developed the ZSON extension [1]. The
>>> extension introduces the new ZSON type, which is 100% compatible with JSONB but
>>> uses a shared dictionary of strings most frequently used in given JSONB
>>> documents for compression. These strings are replaced with integer IDs.
>>> Afterward, PGLZ (and now LZ4) applies if the document is large enough by common
>>> PostgreSQL logic. Under certain conditions (many large documents), this saves
>>> disk space, memory and increases the overall performance. More details can be
>>> found in README on GitHub.
>> I think this is interesting because it is one of the few cases that
>> allow compression outside of a single column. Here is a list of
>> compression options:
>>
>> https://momjian.us/main/blogs/pgblog/2020.html#April_27_2020
>>
>> 1. single field
>> 2. across rows in a single page
>> 3. across rows in a single column
>> 4. across all columns and rows in a table
>> 5. across tables in a database
>> 6. across databases
>>
>> While standard Postgres does #1, ZSON allows 2-5, assuming the data is
>> in the ZSON data type. I think this cross-field compression has great
>> potential for cases where the data is not relational, or hasn't had time
>> to be structured relationally. It also opens questions of how to do
>> this cleanly in a relational system.
>>
>
> I think we're going to get the best bang for the buck on doing 2, 3, and
> 4. If it's confined to a single table then we can put a dictionary in
> something like a fork.
Agreed.
> Maybe given partitioning we want to be able to do multi-table
> dictionaries, but that's less certain.
>
Yeah. I think it'll have many of the same issues/complexity as global
indexes, and the gains are likely limited. At least assuming the
partitions are sufficiently large, but tiny partitions are inefficient
in general, I think.
regards
--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company