PgExtnRegistry API Reference

Overview

PgExtnRegistry is a singleton class that manages PostgreSQL extension metadata and function pointers. It serves as the central registry for all loaded extensions, providing lookup and invocation APIs for extension types, operators, and operator classes. Location: src/pg_ext/extn_registry.cc, include/pg_ext/extn_registry.hh Key Responsibilities:

Load extension shared libraries via dlopen()
Maintain mappings from OIDs and names to function pointers
Provide type conversion utilities (binary ↔ Datum ↔ string)
Support opclass method invocation for GIN/GIST indexes
Enable extension operator comparisons

Core Data Structures

PgType

Represents a PostgreSQL type with its I/O functions:

struct PgType {
    uint32_t oid;           // PostgreSQL type OID
    std::string typinput;   // Text input function name (e.g., "int4in")
    std::string typoutput;  // Text output function name (e.g., "int4out")
    std::string typreceive; // Binary receive function name (e.g., "int4recv")
    std::string typsend;    // Binary send function name (e.g., "int4send")
};

PgOpsClass

Represents an operator class (e.g., gin_trgm_ops, gist_point_ops):

struct PgOpsClass {
    uint32_t oid;                // OpClass OID from pg_opclass
    std::string name;            // OpClass name (e.g., "gin_trgm_ops")
    std::string schema;          // Schema name (e.g., "public")
    std::string access_method;   // Access method: "gin" or "gist"
    std::string family;          // OpFamily name
};

PgOpsClassMethod

Represents a support function for an operator class:

struct PgOpsClassMethod {
    uint32_t input_type_oid;     // Input type OID
    std::string input_type;      // Input type name
    uint32_t key_type_oid;       // Key type OID (for index storage)
    std::string key_type;        // Key type name
    int support_number;          // Support proc number (1-11)
    std::string function_name;   // Function name (e.g., "gin_extract_value_trgm")
    PGFunction function_ptr = nullptr;  // dlsym() loaded function pointer
};

Support Numbers (from constants.hh): GIN:

GIN_COMPARE = 1 - Compare function
GIN_EXTRACTVALUE = 2 - Extract keys from indexed value
GIN_EXTRACTQUERY = 3 - Extract keys from query
GIN_CONSISTENT = 4 - Check if entry matches query
GIN_COMPARE_PARTIAL = 5 - Partial match comparison
GIN_TRICONSISTENT = 6 - Ternary consistency check
GIN_OPTIONS = 7 - Index options

GIST:

GIST_CONSISTENT = 1 - Check consistency
GIST_UNION = 2 - Union/merge predicates
GIST_COMPRESS = 3 - Compress leaf value
GIST_DECOMPRESS = 4 - Decompress value
GIST_PENALTY = 5 - Calculate insertion penalty
GIST_PICKSPLIT = 6 - Split page algorithm
GIST_EQUAL = 7 - Equality check
GIST_DISTANCE = 8 - Distance for KNN
GIST_FETCH = 9 - Fetch tuple
GIST_OPTIONS = 10 - Index options
GIST_SORTSUPPORT = 11 - Sort support

Library Management

init_libraries()

Load an extension shared library and register it.

void init_libraries(uint64_t db_id,
                    const std::string& extension,
                    const std::string& extension_lib_path);

Parameters:

db_id: Database ID
extension: Extension name (e.g., “pg_trgm”)
extension_lib_path: Full path to .so file (e.g., “/usr/lib/postgresql/16/lib/pg_trgm.so”)

Behavior:

Calls dlopen(lib_path, RTLD_NOW | RTLD_GLOBAL)
Stores library handle in _library_map[extension]
Throws error if library cannot be loaded

Example:

auto registry = PgExtnRegistry::get_instance();
registry->init_libraries(16384, "pg_trgm", "/usr/lib/postgresql/16/lib/pg_trgm.so");

Type Management APIs

add_type()

void add_type(const std::string& extension,
              uint32_t oid,
              const std::string& typinput,
              const std::string& typoutput,
              const std::string& typreceive,
              const std::string& typsend);

Parameters:

extension: Extension name
oid: PostgreSQL type OID
typinput: Text input function name
typoutput: Text output function name
typreceive: Binary receive function name
typsend: Binary send function name

Behavior:

Uses dlsym() to load all four I/O functions from extension library
Stores functions in _type_func_name_to_func map
Stores type metadata in _type_oid_to_type map

Example:

registry->add_type("pg_trgm", 16385, "gtrgmin", "gtrrgmout", "gtrgmrecv", "gtrgmsend");

get_type_by_oid()

Retrieve type metadata by OID.

PgType get_type_by_oid(uint32_t oid) const;

Returns: PgType structure or empty struct if not found Example:

PgType type = registry->get_type_by_oid(16385);
std::cout << "Type input function: " << type.typinput << std::endl;

get_type_func_by_type_name()

Get a type I/O function by name.

PGFunction get_type_func_by_type_name(const std::string& type_name) const;

Parameters:

type_name: Function name (e.g., “int4in”, “gtrgmrecv”)

Returns: Function pointer or nullptr if not found

binary_to_datum()

Convert binary wire format to PostgreSQL Datum using typreceive function.

Datum binary_to_datum(const std::span<const char>& value,
                      Oid pg_oid,
                      int32_t atttypmod) const;

Parameters:

value: Binary data from PostgreSQL wire protocol
pg_oid: Type OID
atttypmod: Type modifier (e.g., varchar length)

Returns: Datum representation of the value Implementation:

Datum PgExtnRegistry::binary_to_datum(const std::span<const char>& value,
                                      Oid pg_oid,
                                      int32_t atttypmod) const {
    auto type = get_type_by_oid(pg_oid);
    auto typreceive = get_type_func_by_type_name(type.typreceive);

    // Create StringInfo for binary data
    StringInfoData string;
    initStringInfo(&string);
    appendBinaryStringInfoNT(&string, value.data(), value.size());

    // Call typreceive function
    PGFunction typreceive_func = (PGFunction)typreceive;
    Datum result = DirectFunctionCall3(typreceive_func,
                                      PointerGetDatum(&string),
                                      ObjectIdGetDatum(0),
                                      Int32GetDatum(atttypmod));
    return result;
}

Example:

// Convert binary point data to Datum
std::span<const char> binary_point = get_binary_data();
Datum point_datum = registry->binary_to_datum(binary_point, POINTOID, -1);

datum_to_string()

Convert Datum to string representation using typoutput function.

std::string datum_to_string(Datum value, Oid pg_oid) const;

Parameters:

value: Datum to convert
pg_oid: Type OID

Returns: String representation of the value Implementation:

std::string PgExtnRegistry::datum_to_string(Datum value, Oid pg_oid) const {
    auto type = get_type_by_oid(pg_oid);
    auto typoutput = get_type_func_by_type_name(type.typoutput);

    // Call typoutput function
    PGFunction typoutput_func = (PGFunction)typoutput;
    Datum result = DirectFunctionCall1(typoutput_func, value);
    const char* str = DatumGetCString(result);

    return std::string(str);
}

Example:

// Convert point datum to string "(1.5,2.3)"
std::string point_str = registry->datum_to_string(point_datum, POINTOID);

Operator Management APIs

add_operator()

void add_operator(const std::string& extension,
                  uint32_t oid,
                  const std::string& oper_name,
                  const std::string& oper_proc);

Parameters:

extension: Extension name
oid: Operator OID from pg_operator
oper_name: Operator symbol (e.g., =, <@, &&)
oper_proc: Implementation function name

Behavior:

Uses dlsym() to load operator function from extension library
Stores in _oper_name_to_func and _proc_name_to_func maps
Stores OID mappings in _oper_oid_to_name and _proc_oid_to_name

Example:

registry->add_operator("pg_trgm", 3636, "%", "similarity");

get_operator_func_by_oid()

Get operator function by OID.

PGFunction get_operator_func_by_oid(uint32_t oid) const;

Returns: Function pointer or nullptr if not found

get_operator_func_by_oper_name()

Get operator function by operator symbol.

PGFunction get_operator_func_by_oper_name(const char* oper_name) const;

Parameters:

oper_name: Operator symbol (e.g., =, %, <@)

Returns: Function pointer or nullptr if not found Example:

void* similarity_func = registry->get_operator_func_by_oper_name("%");

get_operator_func_by_proc_name()

Get operator function by procedure name.

PGFunction get_operator_func_by_proc_name(const std::string& proc_name) const;

Example:

void* func = registry->get_operator_func_by_proc_name("similarity");

comparator_func()

Compare two values using an extension operator.

static bool comparator_func(const ExtensionContext* context,
                           const std::span<const char>& lhval,
                           const std::span<const char>& rhval);

Parameters:

context: Contains type_oid and op_str (operator name)
lhval: Left-hand value (binary format)
rhval: Right-hand value (binary format)

Returns: Boolean result of comparison Implementation:

bool PgExtnRegistry::comparator_func(const ExtensionContext* context,
                                     const std::span<const char>& lhval,
                                     const std::span<const char>& rhval) {
    auto extn_registry = PgExtnRegistry::get_instance();

    // Convert binary to Datum
    Datum left_datum = extn_registry->binary_to_datum(lhval, context->type_oid, -1);
    Datum right_datum = extn_registry->binary_to_datum(rhval, context->type_oid, -1);

    // Get operator function
    auto operator_func = extn_registry->get_operator_func_by_oper_name(context->op_str);

    // Invoke operator
    PGFunction operator_func_ptr = (PGFunction)operator_func;
    Datum result = DirectFunctionCall3(operator_func_ptr, left_datum, right_datum,
                                      ObjectIdGetDatum(0));

    return DatumGetBool(result);
}

Example:

// Compare two PostGIS geometries for equality
ExtensionContext ctx;
ctx.type_oid = GEOMETRYOID;
ctx.op_str = "=";

bool equal = PgExtnRegistry::comparator_func(&ctx, geometry1_binary, geometry2_binary);

Operator Class APIs

add_opclass()

void add_opclass(const std::string& extension,
                 PgOpsClass opclass,
                 PgOpsClassMethod method);

Parameters:

extension: Extension name
opclass: OpClass metadata
method: Method metadata including support number and function name

Behavior:

Uses dlsym() to load support function
Stores in nested map: _opclass_function_map[opclass_name][support_number]

Example:

PgOpsClass gin_trgm;
gin_trgm.name = "gin_trgm_ops";
gin_trgm.access_method = "gin";

PgOpsClassMethod extractvalue;
extractvalue.support_number = GIN_EXTRACTVALUE;
extractvalue.function_name = "gin_extract_value_trgm";

registry->add_opclass("pg_trgm", gin_trgm, extractvalue);

get_opclass_method_by_method_name()

Retrieve an opclass method by name and support number.

PgOpsClassMethod get_opclass_method_by_method_name(const std::string& opclass_name,
                                                    int support_number);

Parameters:

opclass_name: OpClass name (e.g., “gin_trgm_ops”)
support_number: Support procedure number (e.g., GIN_EXTRACTVALUE = 2)

Returns: PgOpsClassMethod structure with function pointer, or empty struct if not found Example:

auto method = registry->get_opclass_method_by_method_name("gin_trgm_ops", GIN_EXTRACTVALUE);
if (method.function_ptr) {
    PGFunction func = (PGFunction)method.function_ptr;
    // Use function...
}

get_opclass_method_func_ptr_by_method_name()

Get opclass method function pointer.

PGFunction get_opclass_method_func_ptr_by_method_name(const std::string& opclass_name,
                                                      int support_number) const;

Returns: Function pointer or nullptr if not found Example:

void* func = PgExtnRegistry::get_opclass_method_func_ptr_by_method_name("gist_point_ops",
                                                                        GIST_COMPRESS);

Internal Data Structures

Registry Maps

private:
    // Library handles
    std::unordered_map<std::string, void*> _library_map;
    // extension_name → dlopen() handle

    // Type mappings
    std::unordered_map<uint32_t, PgType> _type_oid_to_type;
    // type_oid → PgType

    std::unordered_map<std::string, void*> _type_func_name_to_func;
    // function_name → function_ptr

    // Operator mappings
    std::unordered_map<uint32_t, std::string> _oper_oid_to_name;
    // operator_oid → operator_name

    std::unordered_map<uint32_t, std::string> _proc_oid_to_name;
    // proc_oid → proc_name

    std::unordered_map<std::string, void*> _oper_name_to_func;
    // operator_name → function_ptr

    std::unordered_map<std::string, void*> _proc_name_to_func;
    // proc_name → function_ptr

    // Opclass mappings
    std::unordered_map<std::string,
                       std::unordered_map<int, PgOpsClassMethod>> _opclass_function_map;
    // opclass_name → (support_number → PgOpsClassMethod)

Usage Patterns

Pattern 1: Type I/O Conversion

// Binary (wire protocol) → Datum → String
std::span<const char> binary_data = ...;
Datum datum = registry->binary_to_datum(binary_data, type_oid, -1);
std::string text = registry->datum_to_string(datum, type_oid);

Pattern 2: Opclass Method Invocation

// Get method, check validity, invoke
auto method = registry->get_opclass_method_by_method_name(opclass, support_num);
if (method.function_ptr) {
    PGFunction func = (PGFunction)method.function_ptr;
    Datum result = DirectFunctionCall1(func, input_datum);
    // Process result...
}

Pattern 3: Extension Operator Comparison

// Setup context
ExtensionContext ctx;
ctx.type_oid = custom_type_oid;
ctx.op_str = "=";

// Compare
bool result = PgExtnRegistry::comparator_func(&ctx, lhs_binary, rhs_binary);

Thread Safety

Current Status: Not thread-safe

dlopen() and dlsym() calls are not synchronized
Registry maps are accessed without locks
Singleton instance is not thread-safe

Recommendations:

Initialize all extensions before starting worker threads
Treat registry as read-only after initialization
Add mutex protection if runtime extension loading is needed

Error Handling

All lookup methods log errors and return nullptr / empty struct on failure:

void* func = registry->get_operator_func_by_oid(invalid_oid);
// Logs: "Failed to find operator function by oid: <oid>"
// Returns: nullptr

Best Practice: Always check for nullptr before using function pointers:

void* func = registry->get_type_func_by_type_name("some_func");
if (!func) {
    // Handle error
    return;
}
PGFunction pg_func = (PGFunction)func;
// Safe to use

Performance Considerations

Function Pointer Caching: Frequently-used functions should be cached at call site
OID Lookups: Type/operator OID lookups involve map lookups; cache results when possible
Datum Conversions: Binary ↔ Datum conversions invoke extension functions; minimize conversions
dlsym() Cost: Symbol lookups are relatively expensive; done once during registration

​PgExtnRegistry API Reference

​Overview

​Core Data Structures

​PgType

​PgOpsClass

​PgOpsClassMethod

​Library Management

​init_libraries()

​Type Management APIs

​add_type()

​get_type_by_oid()

​get_type_func_by_type_name()

​binary_to_datum()

​datum_to_string()

​Operator Management APIs

​add_operator()

​get_operator_func_by_oid()

​get_operator_func_by_oper_name()

​get_operator_func_by_proc_name()

​comparator_func()

​Operator Class APIs

​add_opclass()

​get_opclass_method_by_method_name()

​get_opclass_method_func_ptr_by_method_name()

​Internal Data Structures

​Registry Maps

​Usage Patterns

​Pattern 1: Type I/O Conversion

​Pattern 2: Opclass Method Invocation

​Pattern 3: Extension Operator Comparison

​Thread Safety

​Error Handling

​Performance Considerations

PgExtnRegistry API Reference

Overview

Core Data Structures

PgType

PgOpsClass

PgOpsClassMethod

Library Management

init_libraries()

Type Management APIs

add_type()

get_type_by_oid()

get_type_func_by_type_name()

binary_to_datum()

datum_to_string()

Operator Management APIs

add_operator()

get_operator_func_by_oid()

get_operator_func_by_oper_name()

get_operator_func_by_proc_name()

comparator_func()

Operator Class APIs

add_opclass()

get_opclass_method_by_method_name()

get_opclass_method_func_ptr_by_method_name()

Internal Data Structures

Registry Maps

Usage Patterns

Pattern 1: Type I/O Conversion

Pattern 2: Opclass Method Invocation

Pattern 3: Extension Operator Comparison

Thread Safety

Error Handling

Performance Considerations