Update doc

Signed-off-by: Chen Dai <[email protected]>

docs/index.md

@@ -10,10 +10,11 @@ A Flint index is ...
 
 ### Feature Highlights
 
-- Skipping Index
+- Skipping Index: accelerate data scan by maintaining compact aggregate data structure which includes
   - Partition: skip data scan by maintaining and filtering partitioned column value per file.
   - MinMax: skip data scan by maintaining lower and upper bound of the indexed column per file.
   - ValueSet: skip data scan by building a unique value set of the indexed column per file.
+- Covering Index: create index for selected columns within the source dataset to improve query performance
 
 Please see the following example in which Index Building Logic and Query Rewrite Logic column shows the basic idea behind each skipping index implementation.
 
@@ -117,7 +118,7 @@ High level API is dependent on query engine implementation. Please see Query Eng
 
 ### SQL
 
-DDL statement:
+#### Skipping Index
 
 ```sql
 CREATE SKIPPING INDEX
@@ -157,6 +158,38 @@ DESCRIBE SKIPPING INDEX ON alb_logs
 DROP SKIPPING INDEX ON alb_logs
 ```
 
+#### Covering Index
+
+```sql
+CREATE INDEX name ON <object>
+( column [, ...] )
+WHERE <filter_predicate>
+WITH (auto_refresh = (true|false))
+
+REFRESH INDEX name ON <object>
+
+SHOW INDEX ON <object>
+
+DESCRIBE INDEX name ON <object>
+
+DROP INDEX name ON <object>
+```
+
+Example:
+
+```sql
+CREATE INDEX elb_and_requestUri
+ON alb_logs ( elb, requestUri )
+
+REFRESH INDEX elb_and_requestUri ON alb_logs
+
+SHOW INDEX ON alb_logs
+
+DESCRIBE INDEX elb_and_requestUri ON alb_logs
+
+DROP INDEX elb_and_requestUri ON alb_logs
+```
+
 ## Index Store
 
 ### OpenSearch
@@ -264,6 +297,7 @@ Here is an example for Flint Spark integration:
 ```scala
 val flint = new FlintSpark(spark)
 
+// Skipping index
 flint.skippingIndex()
     .onTable("alb_logs")
     .filterBy("time > 2023-04-01 00:00:00")
@@ -273,6 +307,15 @@ flint.skippingIndex()
     .create()
 
 flint.refresh("flint_alb_logs_skipping_index", FULL)
+
+// Covering index
+flint.coveringIndex()
+    .name("elb_and_requestUri")
+    .onTable("alb_logs")
+    .addIndexColumns("elb", "requestUri")
+    .create()
+
+flint.refresh("flint_alb_logs_elb_and_requestUri_index")
 ```
 
 #### Skipping Index Provider SPI