MEP 6 -- Support vector output fields design

Current state: Accepted

ISSUE: #6299

PRs: #6570 #6598 #6671

Keywords: Query / Search / Vector

Released: Milvus 2.0rc3

Summary

This project is to use minimal memory, let query support to return vector field in output.

Motivation

In Milvus 2.0rc1, operations like query do not support return vector field in output. Proxy will check query request's output fields, if find their data types contain

float vector or binary vector, proxy error out. This is for the consideration of memory consumption,

because vector field with big dimension will occupy hundreds of times of memory comparing with scalar field. So generally load_collection or load_partition

only load scalar fields' raw data into memory. Vector fields' raw data is loaded into memory only in 3 cases:

1. steaming segment
2. vector field's index type is FLAT
3. vector field's index has not been created

If vector's raw data has been loaded into memory, query has already supported to return vector field in output

Design Details

• Add new field VectorFieldInfo into segment struct to return vector field related information  

type VectorFieldInfo struct {
    mu              sync.RWMutex
    fieldID         UniqueID
    fieldBinlog     *datapb.FieldBinlog
    rowDataInMemory bool
    rawData         map[string]storage.FieldData  // map[binlogPath]FieldData
}

type Segment struct {
    ... ...
    vectorFieldInfos map[UniqueID]*VectorFieldInfo
}

• Add new interface in segment_loader

// load vector field's data from info.fieldBinlog, save the raw data into info.rawData
func (loader *segmentLoader) loadSegmentVectorFieldData(info *VectorFieldInfo) error {

• Add new interface in query_collection

// For vector output fields, load raw data from fieldBinlog if needed,
// get vector raw data via result.Offset from *VectorfieldInfo, then
// fill vector raw data into result
func (q *queryCollection) fillVectorFieldsData(segment *Segment, result *segcorepb.RetrieveResults) error

Original vector data storage public interface and struct

Public Interfaces

```go
type FileManager interface {
GetFile(path string) (string, error)
PutFile(path string, content []byte) error
Exist(path string) bool
ReadFile(path string) []byte
}
```

A VectorFileManager implements FileManager interface.

```go
type VectorFileManager struct {
localFileManager FileManager
remoteFileManager FileManager
insertCodec *InsertCodec
}
```

localFileManager is responsible to local file manager. And can be implements with golang os library.
remoteFileManager is responsible for cloud storage or remote server storage, and will be implemented with minio client now.

When the offset of vector is obtained, we can get origin vector data from local vector data file.

Test Plan

Do query / search (with vector field in output fields) in all kinds of combinations of following scenarios, check the correctness of result.

1. float vector or binary vector
2. with/wo index
3. all kinds of index type