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Using `gtfs-via-postgres`, you can import more than one dataset into a single PostgreSQL database by importing them into separate [schemas](https://www.postgresql.org/docs/14/ddl-schemas.html). You can then run queries combine or compare data from them.
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As an example, let's compare two datasets from [Paris](https://en.wikipedia.org/wiki/Île-de-France_Mobilités) and [Berlin](https://en.wikipedia.org/wiki/Verkehrsverbund_Berlin-Brandenburg).
We can now do queries across both datasets, for example finding the geographically furthest 2 stops:
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```sql
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-- warning: takes a long time to compute!
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SELECT
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paris.stop_idAS paris_stop_id,
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berlin.stop_idAS berlin_stop_id
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FROM
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paris.stops paris,
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berlin.stops berlin
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ORDER BYparis.stop_loc<->berlin.stop_locDESC
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LIMIT100
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```
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*Note:* During an import, a function `public.gtfs_via_postgres_import_version()` gets created that returns `gtfs-via-postgres`'s version. If that function already exists (because it has been created by a previous import), its return value is compared to `gtfs-via-postgres`'s version, and if these two versions are not equal, the second import will fail. This ensures that multiple imports into the same database can only be made using the exact same `gtfs-via-postgres` version.
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Using `gtfs-via-postgres`, it is currently *not possible* to import more than one dataset into a single PostgreSQL database.
${opt.schema!=='public' ? `CREATE SCHEMA IF NOT EXISTS "${opt.schema}";` : ''}
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BEGIN;
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-- gtfs-via-postgres supports importing >1 GTFS datasets into 1 DB, each dataset within its own schema. See https://github.com/public-transport/gtfs-via-postgres/issues/51 for more information.
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-- Because almost all helper utilities (enums, functions, etc.) are schema-specific, they get imported more than once. In order to prevent subtle bugs due to incompatibilities among two schemas imported by different gtfs-via-postgres versions, we mock a "mutex" here by checking for public.gtfs_via_postgres_import_version()'s return value.
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-- todo: this can be done more elegantly: just a "DO" block, "ASSERT" that the version matches, create gtfs_via_postgres_import_version() in the "EXCEPTION" block
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CREATE FUNCTION pg_temp.get_gtfs_via_postgres_import_version()
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RETURNS TEXT
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AS $$
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DECLARE
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res TEXT;
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BEGIN
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SELECT public.gtfs_via_postgres_import_version() INTO res;
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RETURN res;
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EXCEPTION
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WHEN undefined_function THEN
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-- do nothing, silence error
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RETURN NULL;
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END;
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$$
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LANGUAGE plpgsql;
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DO $$
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BEGIN
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IF EXISTS (
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SELECT version
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FROM (
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SELECT pg_temp.get_gtfs_via_postgres_import_version() AS version
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) t
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WHERE version != '${pkg.version}'
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) THEN
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RAISE EXCEPTION 'existing GTFS data imported with an incompatible version of gtfs-via-postgres';
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END IF;
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END
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$$
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LANGUAGE plpgsql;
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CREATE OR REPLACE FUNCTION public.gtfs_via_postgres_import_version()
CREATE TYPE "${opt.schema}".exact_times_v AS ENUM (
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CREATE TYPE exact_times_v AS ENUM (
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'frequency_based' -- 0 or empty - Frequency-based trips.
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, 'schedule_based' -- 1 – Schedule-based trips with the exact same headway throughout the day. In this case the end_time value must be greater than the last desired trip start_time but less than the last desired trip start_time + headway_secs.
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);
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CREATE CAST ("${opt.schema}".exact_times_v AS text) WITH INOUT AS IMPLICIT;
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CREATE CAST (exact_times_v AS text) WITH INOUT AS IMPLICIT;
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CREATE TABLE "${opt.schema}".frequencies (
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CREATE TABLE frequencies (
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-- Used to implement arrivals_departures & connections. Filled after COPY-ing, see below.
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