added documentation for external-tables and fallback_schema

Author: vs255034

website/docs/reference/resource-configs/teradata-configs.md

@@ -413,3 +413,89 @@ These steps collectively ensure that the valid_history strategy effectively mana
   )}}
   ```
   See [Collecting Statistics documentation](https://docs.teradata.com/r/76g1CuvvQlYBjb2WPIuk3g/RAyUdGfvREwbO9J0DMNpLw) for more information.
+
+## dbt-external-tables
+* [dbt-external-tables](https://github.com/dbt-labs/dbt-external-tables) are supported with dbt-teradata from dbt-teradata v1.9.3 onwards.
+* Under the hood, dbt-teradata uses the concept of foreign tables to create tables from external sources. More information can be found [here](https://docs.teradata.com/r/Enterprise_IntelliFlex_VMware/SQL-Data-Definition-Language-Syntax-and-Examples/Table-Statements/CREATE-FOREIGN-TABLE)
+* User need to add the dbt-external-tables packages as dependency and can be resolved with `dbt deps` command
+```yaml
+packages:
+  - package: dbt-labs/dbt_external_tables
+    version: [">=0.9.0", "<1.0.0"]
+```
+* User need to add dispatch config for the project to pick the overridden macros from dbt-teradata package
+```yaml
+dispatch:
+  - macro_namespace: dbt_external_tables
+    search_order: ['dbt', 'dbt_external_tables']
+```
+* To define `STOREDAS` and `ROWFORMAT` for in dbt-external tables, one of the below options can be used:
+    * user can use the standard dbt-external-tables config `file_format` and `row_format` respectively
+    * Or user can just add it in `USING` config as mentioned in the Teradata's [documentation](https://docs.teradata.com/r/Enterprise_IntelliFlex_VMware/SQL-Data-Definition-Language-Syntax-and-Examples/Table-Statements/CREATE-FOREIGN-TABLE/CREATE-FOREIGN-TABLE-Syntax-Elements/USING-Clause)
+
+* For external source, which requires authentication, user needs to create authentication object and pass it in `tbl_properties` as `EXTERNAL SECURITY` object.
+  For more information on Authentication object please follow this [link](https://docs.teradata.com/r/Enterprise_IntelliFlex_VMware/SQL-Data-Definition-Language-Syntax-and-Examples/Authorization-Statements-for-External-Routines/CREATE-AUTHORIZATION-and-REPLACE-AUTHORIZATION)
+
+* Sample external sources are provided below as references
+```yaml
+version: 2
+sources:
+  - name: teradata_external
+    schema: "{{ target.schema }}"
+    loader: S3
+
+    tables:
+      - name: people_csv_partitioned
+        external: 
+          location: "/s3/s3.amazonaws.com/dbt-external-tables-testing/csv/"
+          file_format: "TEXTFILE"
+          row_format: '{"field_delimiter":",","record_delimiter":"\n","character_set":"LATIN"}'
+          using: |
+            PATHPATTERN  ('$var1/$section/$var3')
+          tbl_properties: |
+            MAP = TD_MAP1
+            ,EXTERNAL SECURITY  MyAuthObj
+          partitions:
+            - name: section
+              data_type: CHAR(1)
+        columns:
+          - name: id
+            data_type: int
+          - name: first_name
+            data_type: varchar(64)
+          - name: last_name
+            data_type: varchar(64)
+          - name: email
+            data_type: varchar(64)
+```
+
+```yaml
+version: 2
+sources:
+  - name: teradata_external
+    schema: "{{ target.schema }}"
+    loader: S3
+
+    tables:
+      - name: people_json_partitioned
+        external:
+          location: '/s3/s3.amazonaws.com/dbt-external-tables-testing/json/'
+          using: |
+            STOREDAS('TEXTFILE')
+            ROWFORMAT('{"record_delimiter":"\n", "character_set":"cs_value"}')
+            PATHPATTERN  ('$var1/$section/$var3')
+          tbl_properties: |
+            MAP = TD_MAP1
+            ,EXTERNAL SECURITY  MyAuthObj
+          partitions:
+            - name: section
+              data_type: CHAR(1)
+```
+
+## Fallback Schema
+dbt-teradata internally created temporary tables to fetch the metadata of views for manifest and catalog creation. In case if user does not have permission to create tables on the schema they are working on, they can define a fallback_schema(to which they have proper create/drop privileges) in dbt_project.yml as variable.
+
+```yaml
+vars:
+  fallback_schema: <schema-name>
+```