## Which issue does this PR close?
Related to #16756
## Rationale for this change
The existing `sql_dialect.rs` example demonstrates `COPY ... STORED AS
...`, which is actually already fully supported by the standard
`DFParser`.
This PR replaces it with the example from #16756: `CREATE EXTERNAL
CATALOG ... STORED AS ... LOCATION ...` with automatic table discovery.
## What changes are included in this PR?
The first commit updates `dialect.rs` to show that `DFParser` already
handles `COPY ... STORED AS`, making it clear this syntax doesn't need
customization.
Example output from `cargo run --example sql_ops -- dialect`:
```
Query: COPY source_table TO 'file.fasta' STORED AS FASTA
--- Parsing without extension ---
Standard DFParser: Parsed as Statement::CopyTo: COPY source_table TO file.fasta STORED AS FASTA
--- Parsing with extension ---
Custom MyParser: Parsed as MyStatement::MyCopyTo: COPY source_table TO 'file.fasta' STORED AS FASTA
```
The second commit adds a new `custom_sql_parser.rs` example that
implements `CREATE EXTERNAL CATALOG my_catalog STORED AS <format>
LOCATION '<url>'` with automatic table discovery from object storage. It
also removes the old `dialect.rs` example.
## Are these changes tested?
Yes, the new example is runnable with `cargo run --example sql_ops --
custom_sql_parser` and demonstrates the full flow from parsing custom
DDL through registering the catalog to querying discovered tables.
Example output:
```
=== Part 1: Standard DataFusion Parser ===
Parsing: CREATE EXTERNAL CATALOG parquet_testing
STORED AS parquet
LOCATION 'local://workspace/parquet-testing/data'
OPTIONS (
'schema_name' = 'staged_data',
'format.pruning' = 'true'
)
Error: SQL error: ParserError("Expected: TABLE, found: CATALOG at Line: 1, Column: 17")
=== Part 2: Custom Parser ===
Parsing: CREATE EXTERNAL CATALOG parquet_testing
STORED AS parquet
LOCATION 'local://workspace/parquet-testing/data'
OPTIONS (
'schema_name' = 'staged_data',
'format.pruning' = 'true'
)
Target Catalog: parquet_testing
Data Location: local://workspace/parquet-testing/data
Resolved Schema: staged_data
Registered 69 tables into schema: staged_data
Executing: SELECT id, bool_col, tinyint_col FROM parquet_testing.staged_data.alltypes_plain LIMIT 5
+----+----------+-------------+
| id | bool_col | tinyint_col |
+----+----------+-------------+
| 4 | true | 0 |
| 5 | false | 1 |
| 6 | true | 0 |
| 7 | false | 1 |
| 2 | true | 0 |
+----+----------+-------------+
```
## Are there any user-facing changes?
Documentation only. I replaced the `sql_dialect.rs` example with
`custom_sql_parser.rs` and updated the README. No API changes.
Geoffrey Claude