Schema introspection

What it is

The introspect_layer() function fetches a hosted feature layer's JSON definition from ArcGIS Online and records every part of it in a metadata catalog inside the agol_fdw schema: fields and their types, geometry and spatial reference, edit-tracking columns, coded-value and range domains, subtypes, attribute rules (constraint, calculation, validation), relationships, and contingent value groups.

That catalog is the single source of truth the rest of the extension relies on — for generating DDL, building triggers, and reconciling schema drift later.

Why it matters

• Nothing is lost. Domains, subtypes, rules, relationships, and contingent values are all captured — not flattened to plain columns.
• Repeatable. Re-run anytime to detect schema drift between AGOL and your materialized tables.
• Metadata-driven. DDL and triggers are generated from the catalog, so the translation stays consistent.
• SQL-callable. Run it from a SQL client — no separate scripting tooling.

See it

SELECT agol_fdw.introspect_layer(
  layer_url := 'https://services.arcgis.com/.../FeatureServer/0',
  layer_name := 'poles'
);

-- now the catalog holds every field, domain, subtype, rule,
-- relationship, and contingent value group for that layer.

Related

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