[postgis-devel] gzip support for ST_AsMVT

[Martin Davis]

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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