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Overview

This document describes the implementation of operator class (opclass) support in Springtail, enabling the system to handle GIN and GiST secondary indexes in addition to the existing B-tree indexes.
Status: This feature is currently in development on branch SPR-1090-gin-gist-base-3 and has not been merged to main.

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Background

PostgreSQL uses operator classes to define the behavior of indexes for different data types. Each index type (B-tree, GIN, GiST) requires specific support functions identified by support numbers. For example:
  • GIN indexes use functions like extractValue, extractQuery, and consistent
  • GiST indexes use functions like consistent, union, compress, decompress, and penalty
Previously, Springtail only supported B-tree secondary indexes. This implementation extends the system to:
  1. Capture and store operator class metadata from PostgreSQL
  2. Route index operations to the appropriate opclass-specific functions
  3. Provide the foundation for building and maintaining GIN/GiST indexes

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Goals

  • Store opclass (operator class name) for each index column and index_type (btree, gin, gist) for each index
  • Enable dynamic invocation of opclass support functions via OpClassHandler
  • Prepare the indexer infrastructure to handle non-B-tree index types

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Implementation Details

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1. New Data Structures

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OpClassHandler (include/common/constants.hh)

struct OpClassHandler {
    using OpClassFunc = uintptr_t (*)(const std::string& opclass_name,
                                      int support_number,
                                      uintptr_t /*Datum*/ datum);
    OpClassFunc opclass_func = nullptr;
    ExtensionContext context = {};
};
This handler encapsulates a callback for invoking opclass-specific functions. The support_number parameter identifies which support function to call (e.g., GIST_CONSISTENT = 1, GIN_COMPARE = 1).

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Index Type Constants

static constexpr std::string_view INDEX_TYPE_GIN = "gin";
static constexpr std::string_view INDEX_TYPE_GIST = "gist";
static constexpr std::string_view INDEX_TYPE_BTREE = "btree";

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2. Schema Extensions

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Replication Messages (include/pg_repl/pg_repl_msg.hh)

Extended PgMsgSchemaIndexColumn with: 
std::string opclass;  // operator class name (e.g., "tsvector_ops", "int4_ops")
Extended PgMsgIndex with: 
std::string index_type;  // "gin", "gist", or "btree"

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Internal Schema (include/storage/schema.hh)

Extended Index::Column with: 
std::string opclass;
Extended Index with: 
std::string index_type;

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System Tables (include/sys_tbl_mgr/system_tables.hh)

Indexes table - Added column:
ColumnPositionType
OPCLASS6TEXT
IndexNames table - Added column:
ColumnPositionType
INDEX_TYPE8TEXT

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3. PostgreSQL Trigger Updates (scripts/triggers.sql)

Modified the index creation trigger to extract opclass and index type from PostgreSQL system catalogs: 
SELECT
    i.indexrelid AS index_oid,
    i.indclass AS indclass,
    am.amname AS index_type
FROM pg_index i
JOIN pg_class ic ON ic.oid = i.indexrelid
JOIN pg_am am ON am.oid = ic.relam
...

-- Extract opclass for each column
SELECT
    opc.opcname AS opclass
FROM unnest(ind_obj.indkey, ind_obj.indclass)
     WITH ORDINALITY AS u(attnum, opclass_oid, ord)
JOIN pg_opclass opc ON opc.oid = u.opclass_oid
This captures:
  • am.amname: The access method name (btree, gin, gist, brin)
  • opc.opcname: The operator class name for each index column

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4. MutableBTree Extensions (include/storage/mutable_btree.hh)

Extended constructor to accept opclass handler and index type: 
MutableBTree(uint64_t database_id,
             const std::filesystem::path &file,
             const std::vector<uint32_t> &keys,
             ExtentSchemaPtr schema,
             uint64_t xid,
             uint64_t max_extent_size,
             const ExtensionCallback &extension_callback = {},
             const OpClassHandler &opclass_handler = {},
             const std::string_view index_type = constant::INDEX_TYPE_BTREE);
New member variables: 
OpClassHandler _opclass_handler;
std::string_view _index_type;

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5. Table Manager Updates

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MutableTable (include/sys_tbl_mgr/mutable_table.hh)

Extended create_index_root signature: 
MutableBTreePtr create_index_root(
    uint64_t index_id,
    const std::vector<uint32_t>& index_columns,
    const ExtensionCallback& extension_callback = {},
    const OpClassHandler& opclass_handler = {},
    const std::string_view index_type = constant::INDEX_TYPE_BTREE);

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TableMgr (include/sys_tbl_mgr/table_mgr.hh)

Extended get_snapshot_table to accept OpClassHandler: 
MutableTablePtr get_snapshot_table(
    uint64_t db_id,
    uint64_t table_id,
    uint64_t snapshot_xid,
    ExtentSchemaPtr schema,
    const std::vector<Index>& secondary_keys,
    const ExtensionCallback &extension_callback = {},
    const OpClassHandler &opclass_handler = {});

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6. Indexer Changes (src/pg_log_mgr/indexer.cc)

The indexer now branches based on index type: 
if (idx._index_request.index().index_type() == constant::INDEX_TYPE_GIN) {
    //XXX: Build GIN INDEX
} else {
    // Default - btree index builder
    root = mutable_table->create_index_root(index_id, idx_cols,
        {PgExtnRegistry::get_instance()->comparator_func});
    // ... existing B-tree build logic
}
Similar branching exists for:
  • Index invalidation during updates
  • Index population during reconciliation

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7. Protobuf Schema Updates (src/proto/sys_tbl_mgr.proto)

message IndexColumn {
    string name = 1;
    int32 position = 2;
    int32 idx_position = 3;
    string opclass = 4;  // NEW
}

message IndexInfo {
    ...
    string index_type = 10;  // NEW
}

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Data Flow

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Index Creation

PostgreSQL CREATE INDEX
        ↓
DDL Trigger (triggers.sql)
        ↓
Extract: opclass, index_type from pg_opclass, pg_am
        ↓
Replication Message (PgMsgIndex)
        ↓
sys_tbl_mgr::Server::_create_index()
        ↓
Store in IndexNames (index_type) and Indexes (opclass) system tables
        ↓
Indexer loads Index metadata (includes index_type, opclass per column)
        ↓
Indexer builds index based on index_type
        ↓
Create MutableBTree with OpClassHandler
        ↓
For GIN/GiST: invoke opclass methods via OpClassHandler

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Path to completion

The core implementation is complete. The remaining blocker is a build failure in the unit test src/pg_fdw/test/where_test.cc. Issue: The test file imports both table and mutable_table headers simultaneously. These headers have conflicting dependencies—one pulls in custom Springtail extension-related imports while the other includes default PostgreSQL imports, causing symbol conflicts during compilation. Resolution: Avoid using mutable_table in the test. Instead, load the table data directly and use only the Table class for scan operations during testing.