Compressing PostgreSQL JSONB data 12x using cstore_fdw | Hacker News Reader

Compressing PostgreSQL JSONB data 12x using cstore_fdw

citusdata.com

50 pointsmslot16 comments

The chosen benchmark (a customer reviews) table is likely something that benefits tremendously from compressed columnar storage: 1) It has a small number of attributes which are almost always present in the records - that is, a fixed schema. 2) The most of the fields are numeric/date, or text with low cardinality (product_category, etc.) These things respond well to huffman codes and run-length encoding.

This is a use case where JSON shouldn't really ever be used, because the schema is pretty much fixed and highly regular. JSONB records essentially carry the schema definition with them per-record and in this case most of that information is duplicated - hence the blowup in its representation on disk.

While column stores are great for answering analytical queries that require scans over the whole table (like the single query example they show), they aren't as good at transactional queries (like serving webpages).

If I were citus, I'd have written the blog post using a dataset of highly irregular JSON blobs - e.g. log messages from lots of different systems or a big collection of web pages (serialized as json representations of the DOM). Maybe we'll see these "in the coming weeks."

(Ozgun from Citus Data)

Sure, we'd be interested in running more numbers. If you have example data sets in mind, could you share them with us?

For clarification, we picked this data set for several reasons. The data set was real, sizeable, publicly available, and it became highly referenced in PostgreSQL's JSON/JSONB development:

http://www.pgcon.org/2014/schedule/attachments/328_9.4json.p...

http://www.pgcon.org/2014/schedule/attachments/313_xml-hstor...

http://blog.2ndquadrant.com/jsonb-type-performance-postgresq...

http://www.pgcon.org/2014/schedule/attachments/318_pgcon-201...

Can you explain HOW are you compressing the json data? Ex, is it just block-pgzip-compress? Or are you exploding each jsonb-field as a separate file like with normal columns ?

"Note. We currently don't support updating table using DELETE, and UPDATE commands. We also don't support single row inserts.".

Just "COPY TO ..." or "INSERT INTO ... SELECT ..." is available it seems.

(source https://github.com/citusdata/cstore_fdw)

So that's a deal killer. Damn.

The biggest bang for your buck with columnstore indexes is for analytical loads. In a data warehousing situation, these are negligible restrictions.

Your use-case is not representative.

So how do you know what his/her use case is?

I don't know what it is, I only know that this comment prompted the deal killer comment:

>>"Note. We currently don't support updating table using DELETE, and UPDATE commands. We also don't support single row inserts.". Just "COPY TO ..." or "INSERT INTO ... SELECT ..." is available it seems.

>So that's a deal killer. Damn.

This limitation is not a dealbreaker at all in a data warehousing environment, as I explained. Thus, I can assume that their use case is not data warehousing.

Correct. I'm not data warehousing. I'm running a parse-like backend out of Postgres, where I have a lot of duplicated json keys. It would be great to have compression on the json to include this, while maintaining the other advantages of the json column store. However as they're hierarchical and there are many of them columns aren't appropriate (as far as I know but I haven't benchmarked some crazy dynamic solution... json certainly is easier to work with).

I would keep an eye on the columnstore space. I know that Microsoft introduced read-only columnstore indices before introducing updateable ones, and their turnaround on it was 2 years - SQL Server 2012 to 2014.

I'd use that as the outside end of an estimate for when the Postgres team will get updateable columnstore working.

Note, updatetable columnstore sucks everywhere, so I don't think so. It's only for OLAP, not OLTP (parse is oltp).

What may help @azinman is group-compression (ex: bigger pages of rows and compress the whole page, where each json-key can be repeated multiple times in a page since each page may have multiple rows). This is what happens on tokudb,hbse,hypertable,cassandra.

Ostensibly this article is about the wins you can get as far as disk size is concerned by using cstore_fdw, but I can't help but notice they weren't creating any indices on this data.

By creating a simple GIN index on the regular jsonb table, you could dramatically improve those query times. So if your workload doesn't require that you do a table scan for all records then the tradeoff in terms of disk space would more than make up for the perf wins provided by the index.

Does anyone know if cstore supports creating indices of any kind, let alone GIN?

columnstore is supposed to not work with indices (although they have different types like zone-maps)

each column is stored separately on disk, so only the requested column-values are read from disk

This makes slow to select/update/delete a single row(oltp) since it needs to fetch multiple pages (for each column). And makes it fast to do queries regarding data in big size (olap) by doing vectorized query execution and sequential-reads on disk

Do you have any resources describing and explaining the internals of cstore_fdw? Talks, documents, etc - something other than having to read the code (though I'm fine with that as well). I would very much like to understand the underlying mechanism of cstore_fdw.

The best resource is the github page: https://github.com/citusdata/cstore_fdw

There's also a video from SFPUG last year, though it's not great quality: https://www.youtube.com/watch?v=yT34i82o99w

https://github.com/citusdata/cstore_fdw

If you haven't found it already.

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