feat: add cursor statements (#5094)

* feat: add sql parsers for cursor operations

* feat: cursor operator

* feat: implement RecordBatchStreamCursor

* feat: implement cursor storage and execution

* test: add tests

* chore: update docstring

* feat: add a temporary sql rewrite for cast in limit

this issue is described in #5097

* test: add more sql for cursor integration test

* feat: reject non-select query for cursor statement

* refactor: address review issues

* test: add empty result case

* feat: address review comments

src/common/recordbatch/src/cursor.rs

@@ -0,0 +1,173 @@
+// Copyright 2023 Greptime Team
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use futures::StreamExt;
+use tokio::sync::Mutex;
+
+use crate::error::Result;
+use crate::recordbatch::merge_record_batches;
+use crate::{RecordBatch, SendableRecordBatchStream};
+
+struct Inner {
+    stream: SendableRecordBatchStream,
+    current_row_index: usize,
+    current_batch: Option<RecordBatch>,
+    total_rows_in_current_batch: usize,
+}
+
+/// A cursor on RecordBatchStream that fetches data batch by batch
+pub struct RecordBatchStreamCursor {
+    inner: Mutex<Inner>,
+}
+
+impl RecordBatchStreamCursor {
+    pub fn new(stream: SendableRecordBatchStream) -> RecordBatchStreamCursor {
+        Self {
+            inner: Mutex::new(Inner {
+                stream,
+                current_row_index: 0,
+                current_batch: None,
+                total_rows_in_current_batch: 0,
+            }),
+        }
+    }
+
+    /// Take `size` of row from the `RecordBatchStream` and create a new
+    /// `RecordBatch` for these rows.
+    pub async fn take(&self, size: usize) -> Result<RecordBatch> {
+        let mut remaining_rows_to_take = size;
+        let mut accumulated_rows = Vec::new();
+
+        let mut inner = self.inner.lock().await;
+
+        while remaining_rows_to_take > 0 {
+            // Ensure we have a current batch or fetch the next one
+            if inner.current_batch.is_none()
+                || inner.current_row_index >= inner.total_rows_in_current_batch
+            {
+                match inner.stream.next().await {
+                    Some(Ok(batch)) => {
+                        inner.total_rows_in_current_batch = batch.num_rows();
+                        inner.current_batch = Some(batch);
+                        inner.current_row_index = 0;
+                    }
+                    Some(Err(e)) => return Err(e),
+                    None => {
+                        // Stream is exhausted
+                        break;
+                    }
+                }
+            }
+
+            // If we still have no batch after attempting to fetch
+            let current_batch = match &inner.current_batch {
+                Some(batch) => batch,
+                None => break,
+            };
+
+            // Calculate how many rows we can take from this batch
+            let rows_to_take_from_batch = remaining_rows_to_take
+                .min(inner.total_rows_in_current_batch - inner.current_row_index);
+
+            // Slice the current batch to get the desired rows
+            let taken_batch =
+                current_batch.slice(inner.current_row_index, rows_to_take_from_batch)?;
+
+            // Add the taken batch to accumulated rows
+            accumulated_rows.push(taken_batch);
+
+            // Update cursor and remaining rows
+            inner.current_row_index += rows_to_take_from_batch;
+            remaining_rows_to_take -= rows_to_take_from_batch;
+        }
+
+        // If no rows were accumulated, return empty
+        if accumulated_rows.is_empty() {
+            return Ok(RecordBatch::new_empty(inner.stream.schema()));
+        }
+
+        // If only one batch was accumulated, return it directly
+        if accumulated_rows.len() == 1 {
+            return Ok(accumulated_rows.remove(0));
+        }
+
+        // Merge multiple batches
+        merge_record_batches(inner.stream.schema(), &accumulated_rows)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+
+    use datatypes::prelude::ConcreteDataType;
+    use datatypes::schema::{ColumnSchema, Schema};
+    use datatypes::vectors::StringVector;
+
+    use super::*;
+    use crate::RecordBatches;
+
+    #[tokio::test]
+    async fn test_cursor() {
+        let schema = Arc::new(Schema::new(vec![ColumnSchema::new(
+            "a",
+            ConcreteDataType::string_datatype(),
+            false,
+        )]));
+
+        let rbs = RecordBatches::try_from_columns(
+            schema.clone(),
+            vec![Arc::new(StringVector::from(vec!["hello", "world"])) as _],
+        )
+        .unwrap();
+
+        let cursor = RecordBatchStreamCursor::new(rbs.as_stream());
+        let result_rb = cursor.take(1).await.expect("take from cursor failed");
+        assert_eq!(result_rb.num_rows(), 1);
+
+        let result_rb = cursor.take(1).await.expect("take from cursor failed");
+        assert_eq!(result_rb.num_rows(), 1);
+
+        let result_rb = cursor.take(1).await.expect("take from cursor failed");
+        assert_eq!(result_rb.num_rows(), 0);
+
+        let rb = RecordBatch::new(
+            schema.clone(),
+            vec![Arc::new(StringVector::from(vec!["hello", "world"])) as _],
+        )
+        .unwrap();
+        let rbs2 =
+            RecordBatches::try_new(schema.clone(), vec![rb.clone(), rb.clone(), rb]).unwrap();
+        let cursor = RecordBatchStreamCursor::new(rbs2.as_stream());
+        let result_rb = cursor.take(3).await.expect("take from cursor failed");
+        assert_eq!(result_rb.num_rows(), 3);
+        let result_rb = cursor.take(2).await.expect("take from cursor failed");
+        assert_eq!(result_rb.num_rows(), 2);
+        let result_rb = cursor.take(2).await.expect("take from cursor failed");
+        assert_eq!(result_rb.num_rows(), 1);
+        let result_rb = cursor.take(2).await.expect("take from cursor failed");
+        assert_eq!(result_rb.num_rows(), 0);
+
+        let rb = RecordBatch::new(
+            schema.clone(),
+            vec![Arc::new(StringVector::from(vec!["hello", "world"])) as _],
+        )
+        .unwrap();
+        let rbs3 =
+            RecordBatches::try_new(schema.clone(), vec![rb.clone(), rb.clone(), rb]).unwrap();
+        let cursor = RecordBatchStreamCursor::new(rbs3.as_stream());
+        let result_rb = cursor.take(10).await.expect("take from cursor failed");
+        assert_eq!(result_rb.num_rows(), 6);
+    }
+}

src/common/recordbatch/src/error.rs

@@ -168,6 +168,13 @@ pub enum Error {
         #[snafu(source)]
         error: tokio::time::error::Elapsed,
     },
+    #[snafu(display("RecordBatch slice index overflow: {visit_index} > {size}"))]
+    RecordBatchSliceIndexOverflow {
+        #[snafu(implicit)]
+        location: Location,
+        size: usize,
+        visit_index: usize,
+    },
 }
 
 impl ErrorExt for Error {
@@ -182,7 +189,8 @@ impl ErrorExt for Error {
             | Error::Format { .. }
             | Error::ToArrowScalar { .. }
             | Error::ProjectArrowRecordBatch { .. }
-            | Error::PhysicalExpr { .. } => StatusCode::Internal,
+            | Error::PhysicalExpr { .. }
+            | Error::RecordBatchSliceIndexOverflow { .. } => StatusCode::Internal,
 
             Error::PollStream { .. } => StatusCode::EngineExecuteQuery,
 

src/common/recordbatch/src/lib.rs

@@ -15,6 +15,7 @@
 #![feature(never_type)]
 
 pub mod adapter;
+pub mod cursor;
 pub mod error;
 pub mod filter;
 mod recordbatch;

src/common/recordbatch/src/recordbatch.rs

@@ -23,7 +23,7 @@ use datatypes::value::Value;
 use datatypes::vectors::{Helper, VectorRef};
 use serde::ser::{Error, SerializeStruct};
 use serde::{Serialize, Serializer};
-use snafu::{OptionExt, ResultExt};
+use snafu::{ensure, OptionExt, ResultExt};
 
 use crate::error::{
     self, CastVectorSnafu, ColumnNotExistsSnafu, DataTypesSnafu, ProjectArrowRecordBatchSnafu,
@@ -194,6 +194,19 @@ impl RecordBatch {
             .map(|t| t.to_string())
             .unwrap_or("failed to pretty display a record batch".to_string())
     }
+
+    /// Return a slice record batch starts from offset, with len rows
+    pub fn slice(&self, offset: usize, len: usize) -> Result<RecordBatch> {
+        ensure!(
+            offset + len <= self.num_rows(),
+            error::RecordBatchSliceIndexOverflowSnafu {
+                size: self.num_rows(),
+                visit_index: offset + len
+            }
+        );
+        let columns = self.columns.iter().map(|vector| vector.slice(offset, len));
+        RecordBatch::new(self.schema.clone(), columns)
+    }
 }
 
 impl Serialize for RecordBatch {
@@ -256,6 +269,36 @@ impl Iterator for RecordBatchRowIterator<'_> {
     }
 }
 
+/// merge multiple recordbatch into a single
+pub fn merge_record_batches(schema: SchemaRef, batches: &[RecordBatch]) -> Result<RecordBatch> {
+    let batches_len = batches.len();
+    if batches_len == 0 {
+        return Ok(RecordBatch::new_empty(schema));
+    }
+
+    let n_rows = batches.iter().map(|b| b.num_rows()).sum();
+    let n_columns = schema.num_columns();
+    // Collect arrays from each batch
+    let mut merged_columns = Vec::with_capacity(n_columns);
+
+    for col_idx in 0..n_columns {
+        let mut acc = schema.column_schemas()[col_idx]
+            .data_type
+            .create_mutable_vector(n_rows);
+
+        for batch in batches {
+            let column = batch.column(col_idx);
+            acc.extend_slice_of(column.as_ref(), 0, column.len())
+                .context(error::DataTypesSnafu)?;
+        }
+
+        merged_columns.push(acc.to_vector());
+    }
+
+    // Create a new RecordBatch with merged columns
+    RecordBatch::new(schema, merged_columns)
+}
+
 #[cfg(test)]
 mod tests {
     use std::sync::Arc;
@@ -375,4 +418,80 @@ mod tests {
 
         assert!(record_batch_iter.next().is_none());
     }
+
+    #[test]
+    fn test_record_batch_slice() {
+        let column_schemas = vec![
+            ColumnSchema::new("numbers", ConcreteDataType::uint32_datatype(), false),
+            ColumnSchema::new("strings", ConcreteDataType::string_datatype(), true),
+        ];
+        let schema = Arc::new(Schema::new(column_schemas));
+        let columns: Vec<VectorRef> = vec![
+            Arc::new(UInt32Vector::from_slice(vec![1, 2, 3, 4])),
+            Arc::new(StringVector::from(vec![
+                None,
+                Some("hello"),
+                Some("greptime"),
+                None,
+            ])),
+        ];
+        let recordbatch = RecordBatch::new(schema, columns).unwrap();
+        let recordbatch = recordbatch.slice(1, 2).expect("recordbatch slice");
+        let mut record_batch_iter = recordbatch.rows();
+        assert_eq!(
+            vec![Value::UInt32(2), Value::String("hello".into())],
+            record_batch_iter
+                .next()
+                .unwrap()
+                .into_iter()
+                .collect::<Vec<Value>>()
+        );
+
+        assert_eq!(
+            vec![Value::UInt32(3), Value::String("greptime".into())],
+            record_batch_iter
+                .next()
+                .unwrap()
+                .into_iter()
+                .collect::<Vec<Value>>()
+        );
+
+        assert!(record_batch_iter.next().is_none());
+
+        assert!(recordbatch.slice(1, 5).is_err());
+    }
+
+    #[test]
+    fn test_merge_record_batch() {
+        let column_schemas = vec![
+            ColumnSchema::new("numbers", ConcreteDataType::uint32_datatype(), false),
+            ColumnSchema::new("strings", ConcreteDataType::string_datatype(), true),
+        ];
+        let schema = Arc::new(Schema::new(column_schemas));
+        let columns: Vec<VectorRef> = vec![
+            Arc::new(UInt32Vector::from_slice(vec![1, 2, 3, 4])),
+            Arc::new(StringVector::from(vec![
+                None,
+                Some("hello"),
+                Some("greptime"),
+                None,
+            ])),
+        ];
+        let recordbatch = RecordBatch::new(schema.clone(), columns).unwrap();
+
+        let columns: Vec<VectorRef> = vec![
+            Arc::new(UInt32Vector::from_slice(vec![1, 2, 3, 4])),
+            Arc::new(StringVector::from(vec![
+                None,
+                Some("hello"),
+                Some("greptime"),
+                None,
+            ])),
+        ];
+        let recordbatch2 = RecordBatch::new(schema.clone(), columns).unwrap();
+
+        let merged = merge_record_batches(schema.clone(), &[recordbatch, recordbatch2])
+            .expect("merge recordbatch");
+        assert_eq!(merged.num_rows(), 8);
+    }
 }

src/frontend/src/instance.rs

@@ -487,7 +487,11 @@ pub fn check_permission(
         // TODO(dennis): add a hook for admin commands.
         Statement::Admin(_) => {}
         // These are executed by query engine, and will be checked there.
-        Statement::Query(_) | Statement::Explain(_) | Statement::Tql(_) | Statement::Delete(_) => {}
+        Statement::Query(_)
+        | Statement::Explain(_)
+        | Statement::Tql(_)
+        | Statement::Delete(_)
+        | Statement::DeclareCursor(_) => {}
         // database ops won't be checked
         Statement::CreateDatabase(_)
         | Statement::ShowDatabases(_)
@@ -580,6 +584,8 @@ pub fn check_permission(
         Statement::TruncateTable(stmt) => {
             validate_param(stmt.table_name(), query_ctx)?;
         }
+        // cursor operations are always allowed once it's created
+        Statement::FetchCursor(_) | Statement::CloseCursor(_) => {}
     }
     Ok(())
 }