feat: support querying metric engine from frontend (#2987)

* query one logical table

Signed-off-by: Ruihang Xia <waynestxia@gmail.com>

* map column id

Signed-off-by: Ruihang Xia <waynestxia@gmail.com>

* remove deadcode

Signed-off-by: Ruihang Xia <waynestxia@gmail.com>

* fix typo

Signed-off-by: Ruihang Xia <waynestxia@gmail.com>

* remove redundent column name

Signed-off-by: Ruihang Xia <waynestxia@gmail.com>

---------

Signed-off-by: Ruihang Xia <waynestxia@gmail.com>

src/metric-engine/src/engine.rs

@@ -152,7 +152,10 @@ impl RegionEngine for MetricEngine {
 
     /// Retrieves region's metadata.
     async fn get_metadata(&self, region_id: RegionId) -> Result<RegionMetadataRef, BoxedError> {
-        todo!()
+        self.inner
+            .load_region_metadata(region_id)
+            .await
+            .map_err(BoxedError::new)
     }
 
     /// Retrieves region's disk usage.
@@ -261,7 +264,7 @@ mod test {
             .await
             .unwrap_err();
 
-        // open nonexistent region
+        // open nonexistent region won't report error
         let invalid_open_request = RegionOpenRequest {
             engine: METRIC_ENGINE_NAME.to_string(),
             region_dir: env.default_region_dir(),
@@ -274,6 +277,6 @@ mod test {
                 RegionRequest::Open(invalid_open_request),
             )
             .await
-            .unwrap_err();
+            .unwrap();
     }
 }

src/metric-engine/src/engine/alter.rs

@@ -93,12 +93,7 @@ impl MetricEngineInner {
         // register columns to logical region
         for col in columns {
             self.metadata_region
-                .add_column(
+                .add_column(metadata_region_id, region_id, &col.column_metadata)
-                    metadata_region_id,
-                    region_id,
-                    &col.column_metadata.column_schema.name,
-                    col.column_metadata.semantic_type,
-                )
                 .await?;
         }
 

src/metric-engine/src/engine/create.rs

@@ -185,12 +185,7 @@ impl MetricEngineInner {
             .await?;
         for col in &request.column_metadatas {
             self.metadata_region
-                .add_column(
+                .add_column(metadata_region_id, logical_region_id, col)
-                    metadata_region_id,
-                    logical_region_id,
-                    &col.column_schema.name,
-                    col.semantic_type,
-                )
                 .await?;
         }
 
@@ -221,12 +216,7 @@ impl MetricEngineInner {
         // register columns to metadata region
         for col in &new_columns {
             self.metadata_region
-                .add_column(
+                .add_column(metadata_region_id, logical_region_id, col)
-                    metadata_region_id,
-                    logical_region_id,
-                    &col.column_schema.name,
-                    col.semantic_type,
-                )
                 .await?;
         }
 

src/metric-engine/src/engine/open.rs

@@ -52,19 +52,13 @@ impl MetricEngineInner {
             self.open_physical_region(region_id, request).await?;
             self.recover_states(region_id).await?;
 
-            Ok(0)
-        } else if self
-            .state
-            .read()
-            .await
-            .logical_regions()
-            .contains_key(&region_id)
-        {
-            // if the logical region is already open, do nothing
             Ok(0)
         } else {
-            // throw RegionNotFound error
+            // Don't check if the logical region exist. Because a logical region cannot be opened
-            Err(LogicalRegionNotFoundSnafu { region_id }.build())
+            // individually, it is always "open" if its physical region is open. But the engine
+            // can't tell if the logical region is not exist or the physical region is not opened
+            // yet. Thus simply return `Ok` here to ignore all those errors.
+            Ok(0)
         }
     }
 

src/metric-engine/src/engine/put.rs

@@ -288,7 +288,7 @@ mod tests {
         // add columns
         let logical_region_id = env.default_logical_region_id();
         let columns = &["odd", "even", "Ev_En"];
-        let alter_request = test_util::alter_logical_region_add_tag_columns(columns);
+        let alter_request = test_util::alter_logical_region_add_tag_columns(123456, columns);
         engine
             .handle_request(logical_region_id, RegionRequest::Alter(alter_request))
             .await

src/metric-engine/src/engine/read.rs

@@ -12,18 +12,24 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::sync::Arc;
+
+use api::v1::SemanticType;
 use common_query::logical_plan::Expr;
 use common_recordbatch::SendableRecordBatchStream;
 use common_telemetry::{error, info, tracing};
 use datafusion::logical_expr;
 use snafu::{OptionExt, ResultExt};
+use store_api::metadata::{RegionMetadata, RegionMetadataBuilder, RegionMetadataRef};
 use store_api::metric_engine_consts::DATA_SCHEMA_TABLE_ID_COLUMN_NAME;
 use store_api::region_engine::RegionEngine;
 use store_api::storage::consts::ReservedColumnId;
 use store_api::storage::{RegionId, ScanRequest};
 
 use crate::engine::MetricEngineInner;
-use crate::error::{LogicalRegionNotFoundSnafu, MitoReadOperationSnafu, Result};
+use crate::error::{
+    InvalidMetadataSnafu, LogicalRegionNotFoundSnafu, MitoReadOperationSnafu, Result,
+};
 use crate::utils;
 
 impl MetricEngineInner {
@@ -67,17 +73,7 @@ impl MetricEngineInner {
         logical_region_id: RegionId,
         request: ScanRequest,
     ) -> Result<SendableRecordBatchStream> {
-        let physical_region_id = {
+        let physical_region_id = self.get_physical_region_id(logical_region_id).await?;
-            let state = &self.state.read().await;
-            state
-                .get_physical_region_id(logical_region_id)
-                .with_context(|| {
-                    error!("Trying to read an nonexistent region {logical_region_id}");
-                    LogicalRegionNotFoundSnafu {
-                        region_id: logical_region_id,
-                    }
-                })?
-        };
         let data_region_id = utils::to_data_region_id(physical_region_id);
         let request = self
             .transform_request(physical_region_id, logical_region_id, request)
@@ -88,6 +84,38 @@ impl MetricEngineInner {
             .context(MitoReadOperationSnafu)
     }
 
+    pub async fn load_region_metadata(&self, region_id: RegionId) -> Result<RegionMetadataRef> {
+        let is_reading_physical_region = self
+            .state
+            .read()
+            .await
+            .physical_regions()
+            .contains_key(&region_id);
+
+        if is_reading_physical_region {
+            self.mito
+                .get_metadata(region_id)
+                .await
+                .context(MitoReadOperationSnafu)
+        } else {
+            let physical_region_id = self.get_physical_region_id(region_id).await?;
+            self.logical_region_metadata(physical_region_id, region_id)
+                .await
+        }
+    }
+
+    async fn get_physical_region_id(&self, logical_region_id: RegionId) -> Result<RegionId> {
+        let state = &self.state.read().await;
+        state
+            .get_physical_region_id(logical_region_id)
+            .with_context(|| {
+                error!("Trying to read an nonexistent region {logical_region_id}");
+                LogicalRegionNotFoundSnafu {
+                    region_id: logical_region_id,
+                }
+            })
+    }
+
     /// Transform the [ScanRequest] from logical region to physical data region.
     async fn transform_request(
         &self,
@@ -130,9 +158,13 @@ impl MetricEngineInner {
         origin_projection: &[usize],
     ) -> Result<Vec<usize>> {
         // project on logical columns
-        let logical_columns = self
+        let all_logical_columns = self
             .load_logical_columns(physical_region_id, logical_region_id)
             .await?;
+        let projected_logical_names = origin_projection
+            .iter()
+            .map(|i| all_logical_columns[*i].column_schema.name.clone())
+            .collect::<Vec<_>>();
 
         // generate physical projection
         let mut physical_projection = Vec::with_capacity(origin_projection.len());
@@ -142,10 +174,9 @@ impl MetricEngineInner {
             .get_metadata(data_region_id)
             .await
             .context(MitoReadOperationSnafu)?;
-        for logical_proj in origin_projection {
+        for name in projected_logical_names {
-            let column_id = logical_columns[*logical_proj].column_id;
             // Safety: logical columns is a strict subset of physical columns
-            physical_projection.push(physical_metadata.column_index_by_id(column_id).unwrap());
+            physical_projection.push(physical_metadata.column_index_by_name(&name).unwrap());
         }
 
         Ok(physical_projection)
@@ -159,7 +190,9 @@ impl MetricEngineInner {
     ) -> Result<Vec<usize>> {
         let logical_columns = self
             .load_logical_columns(physical_region_id, logical_region_id)
-            .await?;
+            .await?
+            .into_iter()
+            .map(|col| col.column_schema.name);
         let mut projection = Vec::with_capacity(logical_columns.len());
         let data_region_id = utils::to_data_region_id(physical_region_id);
         let physical_metadata = self
@@ -167,13 +200,45 @@ impl MetricEngineInner {
             .get_metadata(data_region_id)
             .await
             .context(MitoReadOperationSnafu)?;
-        for logical_col in logical_columns {
+        for name in logical_columns {
-            let column_id = logical_col.column_id;
             // Safety: logical columns is a strict subset of physical columns
-            projection.push(physical_metadata.column_index_by_id(column_id).unwrap());
+            projection.push(physical_metadata.column_index_by_name(&name).unwrap());
         }
+
         Ok(projection)
     }
+
+    pub async fn logical_region_metadata(
+        &self,
+        physical_region_id: RegionId,
+        logical_region_id: RegionId,
+    ) -> Result<RegionMetadataRef> {
+        let logical_columns = self
+            .load_logical_columns(physical_region_id, logical_region_id)
+            .await?;
+
+        let primary_keys = logical_columns
+            .iter()
+            .filter_map(|col| {
+                if col.semantic_type == SemanticType::Tag {
+                    Some(col.column_id)
+                } else {
+                    None
+                }
+            })
+            .collect::<Vec<_>>();
+
+        let mut logical_metadata_builder = RegionMetadataBuilder::new(logical_region_id);
+        for col in logical_columns {
+            logical_metadata_builder.push_column_metadata(col);
+        }
+        logical_metadata_builder.primary_key(primary_keys);
+        let logical_metadata = logical_metadata_builder
+            .build()
+            .context(InvalidMetadataSnafu)?;
+
+        Ok(Arc::new(logical_metadata))
+    }
 }
 
 #[cfg(test)]
@@ -205,7 +270,8 @@ mod test {
             .unwrap();
 
         // add columns to the first logical region
-        let alter_request = alter_logical_region_add_tag_columns(&["987", "789", "654", "321"]);
+        let alter_request =
+            alter_logical_region_add_tag_columns(123456, &["987", "798", "654", "321"]);
         env.metric()
             .handle_request(logical_region_id, RegionRequest::Alter(alter_request))
             .await
@@ -225,7 +291,7 @@ mod test {
             .await
             .unwrap();
 
-        assert_eq!(scan_req.projection.unwrap(), vec![0, 1, 4, 7, 8, 9, 10]);
+        assert_eq!(scan_req.projection.unwrap(), vec![0, 1, 4, 8, 9, 10, 11]);
         assert_eq!(scan_req.filters.len(), 1);
         assert_eq!(
             scan_req.filters[0],
@@ -242,6 +308,6 @@ mod test {
             .transform_request(physical_region_id, logical_region_id, scan_req)
             .await
             .unwrap();
-        assert_eq!(scan_req.projection.unwrap(), vec![0, 1, 4, 7, 8, 9, 10]);
+        assert_eq!(scan_req.projection.unwrap(), vec![0, 1, 4, 8, 9, 10, 11]);
     }
 }

src/metric-engine/src/engine/region_metadata.rs

@@ -27,34 +27,19 @@ use crate::error::Result;
 impl MetricEngineInner {
     /// Load column metadata of a logical region.
     ///
-    /// The return value is ordered.
+    /// The return value is ordered on [ColumnId].
     pub async fn load_logical_columns(
         &self,
         physical_region_id: RegionId,
         logical_region_id: RegionId,
     ) -> Result<Vec<ColumnMetadata>> {
         // load logical and physical columns, and intersect them to get logical column metadata
-        let logical_columns = self
+        let mut logical_column_metadata = self
             .metadata_region
             .logical_columns(physical_region_id, logical_region_id)
             .await?
             .into_iter()
-            .collect::<HashMap<String, SemanticType>>();
+            .map(|(_, column_metadata)| column_metadata)
-        let physical_columns = self
-            .data_region
-            .physical_columns(physical_region_id)
-            .await?;
-        let mut logical_column_metadata = physical_columns
-            .into_iter()
-            .filter_map(|mut col| {
-                // recover the semantic type of logical columns
-                logical_columns
-                    .get(&col.column_schema.name)
-                    .map(|semantic_type| {
-                        col.semantic_type = *semantic_type;
-                        col
-                    })
-            })
             .collect::<Vec<_>>();
 
         // sort columns on column id to ensure the order

src/metric-engine/src/error.rs

@@ -63,6 +63,14 @@ pub enum Error {
         location: Location,
     },
 
+    #[snafu(display("Failed to deserialize column metadata from {}", raw))]
+    DeserializeColumnMetadata {
+        raw: String,
+        #[snafu(source)]
+        error: serde_json::Error,
+        location: Location,
+    },
+
     #[snafu(display("Failed to decode base64 column value"))]
     DecodeColumnValue {
         #[snafu(source)]
@@ -132,6 +140,12 @@ pub enum Error {
 
     #[snafu(display("Alter request to physical region is forbidden"))]
     ForbiddenPhysicalAlter { location: Location },
+
+    #[snafu(display("Invalid region metadata"))]
+    InvalidMetadata {
+        source: store_api::metadata::MetadataError,
+        location: Location,
+    },
 }
 
 pub type Result<T, E = Error> = std::result::Result<T, E>;
@@ -150,8 +164,10 @@ impl ErrorExt for Error {
 
             MissingInternalColumn { .. }
             | DeserializeSemanticType { .. }
+            | DeserializeColumnMetadata { .. }
             | DecodeColumnValue { .. }
-            | ParseRegionId { .. } => StatusCode::Unexpected,
+            | ParseRegionId { .. }
+            | InvalidMetadata { .. } => StatusCode::Unexpected,
 
             PhysicalRegionNotFound { .. } | LogicalRegionNotFound { .. } => {
                 StatusCode::RegionNotFound

src/metric-engine/src/metadata_region.rs

@@ -22,6 +22,7 @@ use common_recordbatch::util::collect;
 use datafusion::prelude::{col, lit};
 use mito2::engine::MitoEngine;
 use snafu::ResultExt;
+use store_api::metadata::ColumnMetadata;
 use store_api::metric_engine_consts::{
     METADATA_SCHEMA_KEY_COLUMN_INDEX, METADATA_SCHEMA_KEY_COLUMN_NAME,
     METADATA_SCHEMA_TIMESTAMP_COLUMN_NAME, METADATA_SCHEMA_VALUE_COLUMN_INDEX,
@@ -32,7 +33,7 @@ use store_api::region_request::RegionPutRequest;
 use store_api::storage::{RegionId, ScanRequest};
 
 use crate::error::{
-    CollectRecordBatchStreamSnafu, DecodeColumnValueSnafu, DeserializeSemanticTypeSnafu,
+    CollectRecordBatchStreamSnafu, DecodeColumnValueSnafu, DeserializeColumnMetadataSnafu,
     MitoReadOperationSnafu, MitoWriteOperationSnafu, ParseRegionIdSnafu, RegionAlreadyExistsSnafu,
     Result,
 };
@@ -96,16 +97,16 @@ impl MetadataRegion {
         &self,
         physical_region_id: RegionId,
         logical_region_id: RegionId,
-        column_name: &str,
+        column_metadata: &ColumnMetadata,
-        semantic_type: SemanticType,
     ) -> Result<bool> {
         let region_id = utils::to_metadata_region_id(physical_region_id);
-        let column_key = Self::concat_column_key(logical_region_id, column_name);
+        let column_key =
+            Self::concat_column_key(logical_region_id, &column_metadata.column_schema.name);
 
         self.put_if_absent(
             region_id,
             column_key,
-            Self::serialize_semantic_type(semantic_type),
+            Self::serialize_column_metadata(column_metadata),
         )
         .await
     }
@@ -132,7 +133,7 @@ impl MetadataRegion {
         let column_key = Self::concat_column_key(logical_region_id, column_name);
         let semantic_type = self.get(region_id, &column_key).await?;
         semantic_type
-            .map(|s| Self::deserialize_semantic_type(&s))
+            .map(|s| Self::deserialize_column_metadata(&s).map(|c| c.semantic_type))
             .transpose()
     }
 
@@ -143,7 +144,7 @@ impl MetadataRegion {
         &self,
         physical_region_id: RegionId,
         logical_region_id: RegionId,
-    ) -> Result<Vec<(String, SemanticType)>> {
+    ) -> Result<Vec<(String, ColumnMetadata)>> {
         let metadata_region_id = utils::to_metadata_region_id(physical_region_id);
         let region_column_prefix = Self::concat_column_key_prefix(logical_region_id);
 
@@ -154,8 +155,8 @@ impl MetadataRegion {
             }
             // Safety: we have checked the prefix
             let (_, column_name) = Self::parse_column_key(&k)?.unwrap();
-            let semantic_type = Self::deserialize_semantic_type(&v)?;
+            let column_metadata = Self::deserialize_column_metadata(&v)?;
-            columns.push((column_name, semantic_type));
+            columns.push((column_name, column_metadata));
         }
 
         Ok(columns)
@@ -228,13 +229,14 @@ impl MetadataRegion {
         }
     }
 
-    pub fn serialize_semantic_type(semantic_type: SemanticType) -> String {
+    pub fn serialize_column_metadata(column_metadata: &ColumnMetadata) -> String {
-        serde_json::to_string(&semantic_type).unwrap()
+        serde_json::to_string(column_metadata).unwrap()
     }
 
-    pub fn deserialize_semantic_type(semantic_type: &str) -> Result<SemanticType> {
+    pub fn deserialize_column_metadata(column_metadata: &str) -> Result<ColumnMetadata> {
-        serde_json::from_str(semantic_type)
+        serde_json::from_str(column_metadata).with_context(|_| DeserializeColumnMetadataSnafu {
-            .with_context(|_| DeserializeSemanticTypeSnafu { raw: semantic_type })
+            raw: column_metadata,
+        })
     }
 }
 
@@ -411,6 +413,8 @@ impl MetadataRegion {
 
 #[cfg(test)]
 mod test {
+    use datatypes::data_type::ConcreteDataType;
+    use datatypes::schema::ColumnSchema;
     use store_api::region_request::RegionRequest;
 
     use super::*;
@@ -463,26 +467,21 @@ mod test {
     }
 
     #[test]
-    fn test_serialize_semantic_type() {
+    fn test_serialize_column_metadata() {
         let semantic_type = SemanticType::Tag;
-        let expected = "\"Tag\"".to_string();
+        let column_metadata = ColumnMetadata {
+            column_schema: ColumnSchema::new("blabla", ConcreteDataType::string_datatype(), false),
+            semantic_type,
+            column_id: 5,
+        };
+        let expected = "{\"column_schema\":{\"name\":\"blabla\",\"data_type\":{\"String\":null},\"is_nullable\":false,\"is_time_index\":false,\"default_constraint\":null,\"metadata\":{}},\"semantic_type\":\"Tag\",\"column_id\":5}".to_string();
         assert_eq!(
-            MetadataRegion::serialize_semantic_type(semantic_type),
+            MetadataRegion::serialize_column_metadata(&column_metadata),
-            expected
-        );
-    }
-
-    #[test]
-    fn test_deserialize_semantic_type() {
-        let semantic_type = "\"Tag\"";
-        let expected = SemanticType::Tag;
-        assert_eq!(
-            MetadataRegion::deserialize_semantic_type(semantic_type).unwrap(),
             expected
         );
 
-        let semantic_type = "\"InvalidType\"";
+        let semantic_type = "\"Invalid Column Metadata\"";
-        assert!(MetadataRegion::deserialize_semantic_type(semantic_type).is_err());
+        assert!(MetadataRegion::deserialize_column_metadata(semantic_type).is_err());
     }
 
     #[test]
@@ -620,13 +619,17 @@ mod test {
         let logical_region_id = RegionId::new(868, 8390);
         let column_name = "column1";
         let semantic_type = SemanticType::Tag;
-        metadata_region
+        let column_metadata = ColumnMetadata {
-            .add_column(
+            column_schema: ColumnSchema::new(
-                physical_region_id,
-                logical_region_id,
                 column_name,
-                semantic_type,
+                ConcreteDataType::string_datatype(),
-            )
+                false,
+            ),
+            semantic_type,
+            column_id: 5,
+        };
+        metadata_region
+            .add_column(physical_region_id, logical_region_id, &column_metadata)
             .await
             .unwrap();
         let actual_semantic_type = metadata_region
@@ -637,12 +640,7 @@ mod test {
 
         // duplicate column won't be updated
         let is_updated = metadata_region
-            .add_column(
+            .add_column(physical_region_id, logical_region_id, &column_metadata)
-                physical_region_id,
-                logical_region_id,
-                column_name,
-                SemanticType::Field,
-            )
             .await
             .unwrap();
         assert!(!is_updated);

src/metric-engine/src/test_util.rs

@@ -30,7 +30,7 @@ use store_api::region_engine::RegionEngine;
 use store_api::region_request::{
     AddColumn, AlterKind, RegionAlterRequest, RegionCreateRequest, RegionRequest,
 };
-use store_api::storage::RegionId;
+use store_api::storage::{ColumnId, RegionId};
 
 use crate::data_region::DataRegion;
 use crate::engine::MetricEngine;
@@ -156,12 +156,15 @@ impl TestEnv {
 }
 
 /// Generate a [RegionAlterRequest] for adding tag columns.
-pub fn alter_logical_region_add_tag_columns(new_tags: &[&str]) -> RegionAlterRequest {
+pub fn alter_logical_region_add_tag_columns(
+    col_id_start: ColumnId,
+    new_tags: &[&str],
+) -> RegionAlterRequest {
     let mut new_columns = vec![];
     for (i, tag) in new_tags.iter().enumerate() {
         new_columns.push(AddColumn {
             column_metadata: ColumnMetadata {
-                column_id: i as u32,
+                column_id: i as u32 + col_id_start,
                 semantic_type: SemanticType::Tag,
                 column_schema: ColumnSchema::new(
                     tag.to_string(),
@@ -198,7 +201,7 @@ pub fn create_logical_region_request(
             ),
         },
         ColumnMetadata {
-            column_id: 0,
+            column_id: 1,
             semantic_type: SemanticType::Field,
             column_schema: ColumnSchema::new(
                 "greptime_value",
@@ -209,7 +212,7 @@ pub fn create_logical_region_request(
     ];
     for tag in tags {
         column_metadatas.push(ColumnMetadata {
-            column_id: 0,
+            column_id: 2,
             semantic_type: SemanticType::Tag,
             column_schema: ColumnSchema::new(
                 tag.to_string(),
@@ -261,9 +264,10 @@ pub fn row_schema_with_tags(tags: &[&str]) -> Vec<PbColumnSchema> {
     schema
 }
 
-/// Build [Rows] for assembling [RegionPutRequest](store_api::region_request::RegionPutRequest).
+/// Build [Row]s for assembling [RegionPutRequest](store_api::region_request::RegionPutRequest).
 ///
-/// The schema is generated by [row_schema_with_tags].
+/// The schema is generated by [row_schema_with_tags]. `num_tags` doesn't need to be precise,
+/// it's used to determine the column id for new columns.
 pub fn build_rows(num_tags: usize, num_rows: usize) -> Vec<Row> {
     let mut rows = vec![];
     for i in 0..num_rows {

src/store-api/src/metadata.rs

@@ -217,6 +217,13 @@ impl RegionMetadata {
         self.id_to_index.get(&column_id).copied()
     }
 
+    /// Find column index by name.
+    pub fn column_index_by_name(&self, column_name: &str) -> Option<usize> {
+        self.column_metadatas
+            .iter()
+            .position(|col| col.column_schema.name == column_name)
+    }
+
     /// Returns the time index column
     ///
     /// # Panics
@@ -261,6 +268,61 @@ impl RegionMetadata {
         self.primary_key.iter().position(|id| *id == column_id)
     }
 
+    /// Project the metadata to a new one using specified column ids.
+    ///
+    /// [RegionId] and schema version are preserved.
+    pub fn project(&self, projection: &[ColumnId]) -> Result<RegionMetadata> {
+        // check time index
+        ensure!(
+            projection.iter().any(|id| *id == self.time_index),
+            TimeIndexNotFoundSnafu
+        );
+
+        // prepare new indices
+        let indices_to_preserve = projection
+            .iter()
+            .map(|id| {
+                self.column_index_by_id(*id)
+                    .with_context(|| InvalidRegionRequestSnafu {
+                        region_id: self.region_id,
+                        err: format!("column id {} not found", id),
+                    })
+            })
+            .collect::<Result<Vec<_>>>()?;
+
+        // project schema
+        let projected_schema =
+            self.schema
+                .try_project(&indices_to_preserve)
+                .with_context(|_| SchemaProjectSnafu {
+                    origin_schema: self.schema.clone(),
+                    projection: projection.to_vec(),
+                })?;
+
+        // project columns, generate projected primary key and new id_to_index
+        let mut projected_column_metadatas = Vec::with_capacity(indices_to_preserve.len());
+        let mut projected_primary_key = vec![];
+        let mut projected_id_to_index = HashMap::with_capacity(indices_to_preserve.len());
+        for index in indices_to_preserve {
+            let col = self.column_metadatas[index].clone();
+            if col.semantic_type == SemanticType::Tag {
+                projected_primary_key.push(col.column_id);
+            }
+            projected_id_to_index.insert(col.column_id, projected_column_metadatas.len());
+            projected_column_metadatas.push(col);
+        }
+
+        Ok(RegionMetadata {
+            schema: Arc::new(projected_schema),
+            time_index: self.time_index,
+            id_to_index: projected_id_to_index,
+            column_metadatas: projected_column_metadatas,
+            primary_key: projected_primary_key,
+            region_id: self.region_id,
+            schema_version: self.schema_version,
+        })
+    }
+
     /// Checks whether the metadata is valid.
     fn validate(&self) -> Result<()> {
         // Id to name.
@@ -621,6 +683,17 @@ pub enum MetadataError {
         err: String,
         location: Location,
     },
+
+    #[snafu(display("Unexpected schema error during project"))]
+    SchemaProject {
+        origin_schema: SchemaRef,
+        projection: Vec<ColumnId>,
+        location: Location,
+        source: datatypes::Error,
+    },
+
+    #[snafu(display("Time index column not found"))]
+    TimeIndexNotFound { location: Location },
 }
 
 impl ErrorExt for MetadataError {