Constructing GeoJSON in PostgreSQL#

In order to efficiently generate a GeoJSON representation of a vast number of locations, I’m currently experimenting with generating the GeoJSON directly inside a PostgreSQL SQL query using json_build_object() and friends.

Here’s my query so far, which illustrates various new patterns I’ve been learning - including a join against a CTE that extracts a many-to-many table into a format that can be represented in the final output as a JSON array.

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with location_concordances as (
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  select location.id, coalesce(
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    jsonb_agg(ci.authority || ':' || ci.identifier) filter (
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      where ci.authority IS NOT NULL
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    ),
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    '[]'
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  ) as concordances
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  from location
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  left join concordance_location cl on location.id = cl.location_id
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  left join concordance_identifier ci on ci.id = cl.concordanceidentifier_id
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  group by location.id
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)
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SELECT
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  json_build_object(
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    'type', 'Feature',
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    'properties', json_build_object(
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      'id',
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      location.public_id,
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      'name',
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      location.name,
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      'state', state.abbreviation,
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      'latitude', location.latitude,
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      'longitude', location.longitude,
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      'location_type', location_type.name,
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      'import_ref', location.import_ref,
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      'phone_number', location.phone_number,
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      'full_address', location.full_address,
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      'city', location.city,
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      'county', county.name,
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      'google_places_id', location.google_places_id,
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      'vaccinefinder_location_id', location.vaccinefinder_location_id,
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      'vaccinespotter_location_id', location.vaccinespotter_location_id,
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      'zip_code', location.zip_code,
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      'hours', location.hours,
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      'website', location.website,
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      'preferred_contact_method', location.preferred_contact_method,
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      'provider', case
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        when provider.name is null then null
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        else jsonb_build_object('name', provider.name, 'type', provider_type.name)
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      end,
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      'concordances', location_concordances.concordances
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    ),
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    'geometry', json_build_object(
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      'type', 'Point',
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      'coordinates', json_build_array(location.longitude, location.latitude)
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    )
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  )
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  from location
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    join state on location.state_id = state.id
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    join county on location.county_id = county.id
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    join location_type on location.location_type_id = location_type.id
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    join location_concordances on location.id = location_concordances.id
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    left join provider on location.provider_id = provider.id
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    left join provider_type on provider.provider_type_id = provider_type.id
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limit 1

Example output from this query:

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{
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    "type": "Feature",
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    "properties": {
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        "id": "rec00NpJzUnVDpLaQ",
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        "name": "Kaiser Permanente Pharmacy #568",
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        "state": "CA",
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        "latitude": 34.081292,
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        "longitude": -117.996576,
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        "location_type": "Pharmacy",
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        "import_ref": "vca-airtable:rec00NpJzUnVDpLaQ",
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        "phone_number": "833-480-4700",
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        "full_address": "12761 Schabarum Ave Plaza Level RM 1100, Irwindale, CA 91706",
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        "city": null,
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        "county": "Los Angeles",
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        "google_places_id": "ChIJizKuijvXwoAR4VAXM2Ek4Nc",
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        "vaccinefinder_location_id": null,
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        "vaccinespotter_location_id": null,
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        "zip_code": null,
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        "hours": "Monday - Friday: 8:00 AM – 5:00 PM\nSaturday - Sunday: Closed",
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        "website": null,
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        "preferred_contact_method": null,
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        "provider": {
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            "name": "Kaiser Permanente",
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            "type": "Health Plan"
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        },
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        "concordances": [
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            "google_places:ChIJizKuijvXwoAR4VAXM2Ek4Nc"
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        ]
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    },
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    "geometry": {
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        "type": "Point",
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        "coordinates": [
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            -117.996576,
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            34.081292
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        ]
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    }
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}

Using ST_AsGeoJSON#

After writing this TIL I found a much quicker option: the ST_AsGeoJSON() function. This works if your table has a geometry column on it - in my case I enhanced my location table to include a point column.

The following SQL generates a full GeoJSON feature row for each location in the database:

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select ST_AsGeoJSON(location.*) from location

The location.* is required - if you try to use ST_AsGeoJSON(*) you get an error claiming that “No function matches the given name and argument types”.

The output looks something like this:

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{
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  "type": "Feature",
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  "geometry": {
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    "type": "Point",
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    "coordinates": [
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      -117.65499,
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      34.10835
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    ]
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  },
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  "properties": {
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    "id": 4285,
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    "name": "Vons Pharmacy #2681",
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    "public_id": "recbECvQlveAyBorV",
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    "import_json": {
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      "Name": "Vons Pharmacy #2681",
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      "Hours": "Monday - Friday: 8:00 AM \u2013 8:00 PM\nSaturday - Sunday: 9:00 AM \u2013 5:00 PM",
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      "County": "San Bernardino County",
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      "county_id": [
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        "reclZ8DWOEuoluStG"
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      ]
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    }
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  }
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}

That import_json section is a PostgreSQL JSON column in the database - note that it gets output as nested data inside the "properties" object.

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Constructing GeoJSON in PostgreSQL#

Using ST_AsGeoJSON#