Add msar.get_schemas DB function

db/schemas/operations/select.py

@@ -3,6 +3,11 @@
 from db.constants import INTERNAL_SCHEMAS
 from db.utils import get_pg_catalog_table
 from db.metadata import get_empty_metadata
+from db.connection import exec_msar_func
+
+
+def get_schemas(conn):
+    return exec_msar_func(conn, 'get_schemas').fetchone()[0]
 
 
 def reflect_schema(engine, name=None, oid=None, metadata=None):

db/sql/0_msar.sql

@@ -662,6 +662,45 @@ SELECT EXISTS (SELECT 1 FROM pg_attribute WHERE attrelid=tab_id AND attname=col_
 $$ LANGUAGE SQL RETURNS NULL ON NULL INPUT;
 
 
+CREATE OR REPLACE FUNCTION msar.get_schemas() RETURNS jsonb AS $$/*
+Return a json array of objects describing the user-defined schemas in the database.
+
+PostgreSQL system schemas are ignored.
+
+Internal Mathesar-specifc schemas are INCLUDED. These should be filtered out by the caller. This
+behavior is to avoid tight coupling between this function and other SQL files that might need to
+define additional Mathesar-specific schemas as our codebase grows.
+
+Each returned JSON object in the array will have the form:
+  {
+    "oid": <int>
+    "name": <str>
+    "description": <str|null>
+    "table_count": <int>
+  }
+*/
+SELECT jsonb_agg(schema_data)
+FROM (
+  SELECT 
+    s.oid AS oid,
+    min(s.nspname) AS name,
+    min(d.description) AS description,
+    COALESCE(count(c.oid), 0) AS table_count
+  FROM pg_catalog.pg_namespace s
+  LEFT JOIN pg_catalog.pg_description d ON
+    d.objoid = s.oid AND
+    d.objsubid = 0
+  LEFT JOIN pg_catalog.pg_class c ON
+    c.relnamespace = s.oid AND
+    c.relkind = 'r'
+  WHERE
+    s.nspname <> 'information_schema' AND
+    s.nspname NOT LIKE 'pg_%'
+  GROUP BY s.oid
+) AS schema_data;
+$$ LANGUAGE sql;
+
+
 ----------------------------------------------------------------------------------------------------
 ----------------------------------------------------------------------------------------------------
 -- ROLE MANIPULATION FUNCTIONS

db/sql/test_0_msar.sql

@@ -2557,3 +2557,54 @@ BEGIN
   );
 END;
 $$ LANGUAGE plpgsql;
+
+
+CREATE OR REPLACE FUNCTION test_get_schemas() RETURNS SETOF TEXT AS $$
+DECLARE
+  initial_schema_count int;
+  foo_schema jsonb;
+BEGIN
+  -- Get the initial schema count
+  SELECT jsonb_array_length(msar.get_schemas()) INTO initial_schema_count;
+
+  -- Create a schema
+  CREATE SCHEMA foo;
+  -- We should now have one additional schema
+  RETURN NEXT is(jsonb_array_length(msar.get_schemas()), initial_schema_count + 1);
+  -- Reflect the "foo" schema
+  SELECT jsonb_path_query(msar.get_schemas(), '$[*] ? (@.name == "foo")') INTO foo_schema;
+  -- We should have a foo schema object
+  RETURN NEXT is(jsonb_typeof(foo_schema), 'object');
+  -- It should have no description
+  RETURN NEXT is(jsonb_typeof(foo_schema->'description'), 'null');
+  -- It should have no tables
+  RETURN NEXT is((foo_schema->'table_count')::int, 0);
+
+  -- And comment
+  COMMENT ON SCHEMA foo IS 'A test schema';
+  -- Create two tables
+  CREATE TABLE foo.test_table_1 (id serial PRIMARY KEY);
+  CREATE TABLE foo.test_table_2 (id serial PRIMARY KEY);
+  -- Reflect again
+  SELECT jsonb_path_query(msar.get_schemas(), '$[*] ? (@.name == "foo")') INTO foo_schema;
+  -- We should see the description we set
+  RETURN NEXT is(foo_schema->'description'#>>'{}', 'A test schema');
+  -- We should see two tables
+  RETURN NEXT is((foo_schema->'table_count')::int, 2);
+
+  -- Drop the tables we created
+  DROP TABLE foo.test_table_1;
+  DROP TABLE foo.test_table_2;
+  -- Reflect the "foo" schema
+  SELECT jsonb_path_query(msar.get_schemas(), '$[*] ? (@.name == "foo")') INTO foo_schema;
+  -- The "foo" schema should now have no tables
+  RETURN NEXT is((foo_schema->'table_count')::int, 0);
+
+  -- Drop the "foo" schema
+  DROP SCHEMA foo;
+  -- We should now have no "foo" schema
+  RETURN NEXT ok(NOT jsonb_path_exists(msar.get_schemas(), '$[*] ? (@.name == "foo")'));
+  -- We should see the initial schema count again
+  RETURN NEXT is(jsonb_array_length(msar.get_schemas()), initial_schema_count);
+END;
+$$ LANGUAGE plpgsql;