Create README.md

README.md

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+# Datafusion-Bigtable
+Bigtable data source for [Apache Arrow Datafusion](https://github.com/apache/arrow-datafusion)
+
+## Run SQL on Bigtable
+
+This crate implements Bigtable data source and Executor for Datafusion. It is built on top of gRPC client [tonic](https://github.com/hyperium/tonic).
+
+## Quick Start
+
+```
+let bigtable_datasource = BigtableDataSource::new(
+    "emulator".to_owned(),                               // project
+    "dev".to_owned(),                                    // instance
+    "weather_balloons".to_owned(),                       // table
+    "measurements".to_owned(),                           // column family
+    vec!["_row_key".to_owned()],                         // table_partition_cols
+    vec![Field::new("pressure", DataType::Utf8, false)], // qualifiers
+    true,                                                // only_read_latest
+).await.unwrap();
+
+let mut ctx = ExecutionContext::new();
+ctx.register_table("weather_balloons", Arc::new(bigtable_datasource)).unwrap();
+
+ctx.sql("SELECT \"_row_key\", pressure, \"_timestamp\" FROM weather_balloons where \"_row_key\" = 'us-west2#3698#2021-03-05-1200'").await?.collect().await?;
+```
+
+## Roadmap
+
+### SQL
+- ✅ select by `"_row_key" =`
+- [ ] select by `"_row_key" IN`
+- [ ] select by `"_row_key" BETWEEN`
+- [ ] select by composite row keys (via `table_partition_cols` and `table_partition_separator`)
+- [ ] Projection pushdown
+- [ ] Predicate push down ([Value range](https://cloud.google.com/bigtable/docs/using-filters#value-range))
+- [ ] Limit Pushdown
+
+### General
+- [ ] Multi Thread or Partition aware execution
+- [ ] Production ready Bigtable SDK in Rust
+
+Note: datafusion-bigtable provides the physical Executor for Datafusion. Any aggregation, group by, join are implemented and handled by Datafusion.