Add example for building an external index for parquet filtes

Author: alamb

datafusion-examples/README.md

@@ -60,6 +60,7 @@ cargo run --example csv_sql
 - [`function_factory.rs`](examples/function_factory.rs): Register `CREATE FUNCTION` handler to implement SQL macros
 - [`make_date.rs`](examples/make_date.rs): Examples of using the make_date function
 - [`memtable.rs`](examples/memtable.rs): Create an query data in memory using SQL and `RecordBatch`es
+- ['parquet_index.rs'](examples/parquet_index.rs): Create an index several parquet files and use it to speed up queries
 - [`parquet_sql.rs`](examples/parquet_sql.rs): Build and run a query plan from a SQL statement against a local Parquet file
 - [`parquet_sql_multiple_files.rs`](examples/parquet_sql_multiple_files.rs): Build and run a query plan from a SQL statement against multiple local Parquet files
 - ['parquet_exec_visitor.rs'](examples/parquet_exec_visitor.rs): Extract statistics by visiting an ExecutionPlan after execution