[postgis-devel] gzip support for ST_AsMVT

Yuri Astrakhan yuriastrakhan at gmail.com
Mon Nov 4 09:21:43 PST 2019 

looks good, thanks again for the blazing fast reply & fix!  I wonder how
soon the demand will bring it into core PG :)

On Mon, Nov 4, 2019 at 11:19 AM Paul Ramsey <pramsey at cleverelephant.ca>
wrote:

> Text as a column can be quite unhappy if you feed it with binary data
> (things like the printing code gets unhappy with the zero-byte parts, so
> you can end up with unexpected stops in printouts).
> I have no idea why text would have any advantage over bytea, since if you
> check the source, they are both just typedefs over top of varlena.
> It would be possible to do a gzip(text) returns bytea, but you’d still
> have a bytea in the end, and it wouldn’t be nice and bidirectional, since
> text::bytea works, but bytea::text doesn’t return the original ascii
> encoding.
>
> There shouldn’t be any conversion cost in a cast, since the memory
> structure of text and bytea is identical, it’s just re-typing a pointer.
>
> I can add a gzip(text) => bytea very easily though (heck, you can, just
> create a new gzip function that looks exactly like the bytea one, but takes
> text as the argument, it’ll work fine.
>
> ATB
>
> P
>
> On Nov 4, 2019, at 1:49 AM, Yuri Astrakhan <yuriastrakhan at gmail.com>
> wrote:
>
> Paul, one (possibly newbie) question - I noticed you used `gzip(bytea)`,
> and I read in [1] that text performs about 17% better (for read queries,
> not sure about in-memory operations). Does it make sense to add gzip(text)
> variant?   I concatenate ST_AsMVT() data with STRING_AGG, thus end up with
> a TEXT, and wonder if there is a conversion cost.  Thanks!!
>
> [1] http://engineering.pivotal.io/post/bytea_versus_text_in_postgresql/
>
> On Mon, Nov 4, 2019 at 4:24 AM Yuri Astrakhan <yuriastrakhan at gmail.com>
> wrote:
>
>> Paul rulez!!!! Thank you!!! :)
>>
>> Experimentation ensues...
>>
>> On Sun, Nov 3, 2019 at 9:29 PM Paul Ramsey <pramsey at cleverelephant.ca>
>> wrote:
>>
>>> Excuse me,
>>>
>>> https://github.com/pramsey/pgsql-gzip
>>>
>>> P
>>>
>>> On Sun, Nov 3, 2019 at 6:24 PM Paul Ramsey <pramsey at cleverelephant.ca>
>>> wrote:
>>> >
>>> > 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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