[postgis-devel] gzip support for ST_AsMVT

[Paul Ramsey]

I could not find a gzip extension on the web, even though I swear one
already exists.
So, I wrote up one, it's quite a small piece of work, but hopefully it serves.

https://github.com/pramsey/psql-gzip

ATB,

P

On Sun, Nov 3, 2019 at 11:57 AM Yuri Astrakhan <yuriastrakhan at gmail.com> wrote:
>
> OSM -> PostgreSQL is done by Imposm 3, which updates database tables on the daily/hourly/minute basis. Every time it runs, it generates a list of changed tiles.  I do not know what process they use for it -- from my perspective, I simply get the list of updated tiles on zoom 14 as a file, and I could use it to regenerate cached tiles or purge them from Varnish.  Perhaps Imposm maintainers could find a good use for that functionality?
>
> [1] https://imposm.org/docs/imposm3/latest/tutorial.html#expire-tiles
>
> On Sun, Nov 3, 2019 at 2:41 PM Martin Davis <mtnclimb at gmail.com> wrote:
>>
>> Good background to know.  Quite a bit to grok there!
>>
>> One question: does any part of that toolchain regenerate tile cache subsets depending on detection of feature change?  And if so, would it be useful to have a DB function which can determine the set of tile ids that need to be refreshed (i.e. by mapping a (set of or single) geometry (or envelopes) into a set of tile ids?
>>
>> On Sun, Nov 3, 2019 at 11:00 AM Yuri Astrakhan <yuriastrakhan at gmail.com> wrote:
>>>
>>> Martin, I am working on improving OpenMapTiles tooling [1] - the ultimate goal is to have tiles generated in real time from the up-to-date OSM data, and serve them directly to user's browser via some caching layer (i.e. Varnish). The tools already contain postserve - a simple python server that queries for MVT tiles (no compression yet, but can be easily added)
>>>
>>> The other task is tile pre-generation using tilelive-copy (nodejs) - I wrote a tilelive-pgquery plugin [2] that queries PG for the tile, compresses it, and passes it on to tilelive-copy for storage.
>>>
>>> [1] OpenMapTiles tools - https://github.com/openmaptiles/openmaptiles-tools
>>> [2] tilelive-pgquery - https://www.npmjs.com/package/tilelive-pgquery
>>>
>>> On Sun, Nov 3, 2019 at 12:55 PM Martin Davis <mtnclimb at gmail.com> wrote:
>>>>
>>>> Great to hear that ST_AsMVT is useful.
>>>>
>>>> The other PostGIS capability that is useful for web spatial applications is the (recently enhanced) ST_AsGeoJSON.  This should also be gzipped over the wire.  So this suggests a modular gzip capability would be more useful.
>>>>
>>>> If this isn't provided in Postgres in some way (now or in near term) perhaps we should just add a ST_Gzip function to PostGIS.
>>>>
>>>> Out of curiosity, what platform do you use for your external gzipping layer?
>>>>
>>>> On Sun, Nov 3, 2019 at 8:29 AM nyurik <yuriastrakhan at gmail.com> wrote:
>>>>>
>>>>> The amazing ST_AsMVT() has two common usage patterns:  copy resulting MVTs to
>>>>> a tile cache (e.g. .mbtiles file or a materialized view), or serve MVT to
>>>>> the users (direct SQL->browser approach).  Both patterns still require one
>>>>> additional data processing step -- gziping.
>>>>>
>>>>> Thus, rather than having a horizontally scalable db plus a simple IO-bound
>>>>> SQL->Web or a SQL->store process, one has to add a relatively CPU-intensive
>>>>> gzipping layer.  This is especially relevant if I try to create a PG table
>>>>> with the pre-generated tiles - I must use an external data compression
>>>>> process to retrieve a tile, gzip it, and store it back, instead of running a
>>>>> single query for copying all tiles.  My cursory look at the tile sizes
>>>>> indicate gzipping shrinks MVTs 50% to 300%.
>>>>>
>>>>> Note that a similar CPU-intensive step - creating MD5 tile hashes for a more
>>>>> efficient storage - can be easily done with PG's `md5()` function, whereas
>>>>> `gzip()` doesn't appear to exist.
>>>>>
>>>>> I would like to propose two possible solutions:
>>>>> * Implement ST_AsMVT(..., compress) parameter - NULL=no compression,
>>>>> 0-9=compression level.
>>>>>    PROs:  adds just the required functionality to where it is needed (YAGNI
>>>>> principle), does not require ungzip yet (ST_AsMVT is a one way function
>>>>> without the corresponding MVT->Table method)
>>>>>    CONs: less generic (unusable for non-MVT usage)
>>>>> * Implement gzip() or ST_gzip()
>>>>>    PROs:  a more generic approach not tied to MVTs
>>>>>    CONs:  logically implies the need of ungzip(), requires PG community to
>>>>> agree this functionality is needed
>>>>>
>>>>> Thanks!
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Sent from: http://postgis.17.x6.nabble.com/PostGIS-Dev-f3570762.html
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>>>>
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