Architecture Overview

This page gives a bird’s-eye view of ProvSQL’s internals: how the extension is loaded, how its components are organized, and how data flows from an SQL query to a provenance evaluation result. For a detailed walkthrough of the query rewriting pipeline, see Query Rewriting Pipeline.

Extension Lifecycle

ProvSQL is a PostgreSQL shared-library extension. Because it installs a planner hook, the library must be loaded at server start via the shared_preload_libraries configuration variable; it cannot be loaded on demand.

When PostgreSQL starts, it calls _PG_init, which:

1. Registers four GUC (Grand Unified Configuration) variables:

   • provsql.active – enable/disable provenance tracking (default: on).

   • provsql.where_provenance – enable where-provenance (default: off).

   • provsql.update_provenance – track provenance through DML statements (default: off).

   • provsql.verbose_level – verbosity for debug messages (0–100, default: 0).

2. Installs the planner hook (provsql_planner) by saving the previous hook in prev_planner and replacing planner_hook.

3. Installs shared-memory hooks for inter-process coordination (see Memory Management).

4. Launches the mmap background worker that manages persistent circuit storage.

When the server shuts down, _PG_fini restores the previous planner and shared-memory hooks.

Component Map

ProvSQL is a mixed C/C++ codebase. The PostgreSQL interface layer is written in C (required by the extension API); complex data structures and algorithms are in C++.

C files (PostgreSQL interface layer):

Planner hook and query rewriting

• provsql.c – planner hook and the bulk of the query rewriting logic (~3400 lines).

Utilities and shared state

• provsql_utils.c / provsql_utils.h – OID cache (get_constants), type helpers, gate-type enum.

• provsql_error.h – provsql_error / _warning / _notice / _log macros.

• c_cpp_compatibility.h – small shims for mixing C and C++ sources.

Memory-mapped circuit store

• provsql_mmap.c / provsql_mmap.h – background worker and IPC primitives.

• provsql_shmem.c / provsql_shmem.h – shared-memory segment setup.

SQL-callable functions

• provenance.c – error stub for the provenance() SQL function (reached only when a query bypasses the planner hook).

• provenance_evaluate.c – SQL-level semiring evaluation (user-defined plus/times/… functions).

• aggregation_evaluate.c – aggregate evaluation entry point.

• agg_token.c / agg_token.h – the agg_token composite type (UUID + running value).

PostgreSQL version compatibility

• compatibility.c / compatibility.h – shims for cross-version PostgreSQL API differences.

C++ files (data structures and algorithms):

Circuit representation

• Circuit.h / Circuit.hpp – template base class parameterised by gate type; inherited by all circuit variants.

• GenericCircuit.h / GenericCircuit.hpp / GenericCircuit.cpp – semiring-agnostic in-memory circuit.

• BooleanCircuit.h / BooleanCircuit.cpp – Boolean circuit used for knowledge compilation and probability evaluation.

• WhereCircuit.h / WhereCircuit.cpp – where-provenance circuit.

• DotCircuit.h / DotCircuit.cpp – GraphViz DOT export of circuits.

Persistent storage and in-memory reconstruction

• MMappedCircuit.h / MMappedCircuit.cpp – mmap-backed persistent circuit store.

• CircuitFromMMap.h / CircuitFromMMap.cpp – reads the mmap store to build in-memory GenericCircuit / BooleanCircuit instances.

• CircuitCache.h / CircuitCache.cpp / circuit_cache.h – per-session gate cache.

• MMappedUUIDHashTable.h / MMappedUUIDHashTable.cpp – open-addressing hash table keyed by UUID, stored in mmap.

• MMappedVector.h / MMappedVector.hpp – std::vector-like container over an mmap region.

Semiring evaluation

• semiring/*.h – header-only semiring implementations (Boolean, Counting, Formula, Why, BoolExpr).

• provenance_evaluate_compiled.cpp / provenance_evaluate_compiled.hpp – dispatcher for compiled semirings.

• having_semantics.cpp / having_semantics.hpp – pre-evaluation of HAVING sub-circuits before the main semiring traversal.

Aggregation

• Aggregation.h / Aggregation.cpp – aggregate operator enum, accumulator interface, and built-in accumulators (see Aggregation Provenance).

Probability, Shapley, knowledge compilation

• probability_evaluate.cpp – probability-method dispatcher (see Probability Evaluation).

• dDNNF.h / dDNNF.cpp – d-DNNF data structure and linear-time probability evaluation.

• dDNNFTreeDecompositionBuilder.h / dDNNFTreeDecompositionBuilder.cpp – constructs a d-DNNF from a tree decomposition.

• TreeDecomposition.h / TreeDecomposition.cpp – tree decomposition via min-fill elimination.

• TreeDecompositionKnowledgeCompiler.cpp – the standalone tdkc binary built by make tdkc.

• shapley.cpp – Shapley and Banzhaf value computation.

Export and visualization

• view_circuit.cpp – SQL view_circuit function (renders a DOT graph via graph-easy).

• to_prov.cpp – PROV-XML export.

• where_provenance.cpp – SQL where-provenance output function.

C++ utilities

• provsql_utils_cpp.h / provsql_utils_cpp.cpp – C++ counterparts to provsql_utils.h, including UUID string conversion.

• subset.cpp / subset.hpp – subset enumeration used by HAVING evaluation.

• Graph.h – lightweight graph helpers used by the tree decomposition code.

• PermutationStrategy.h – pluggable vertex-ordering heuristic for tree decomposition.

• flat_map.hpp / flat_set.hpp – contiguous associative containers used inside hot loops.

Data Flow

The end-to-end flow of a query through ProvSQL:

1. The user submits an SQL query. PostgreSQL parses it into a Query tree and calls the planner.

2. provsql_planner intercepts the call. If the query touches provenance-tracked tables (detected by has_provenance), it calls process_query to rewrite it.

3. The rewritten query carries an extra UUID expression in its target list. When the executor evaluates the query, it calls ProvSQL’s SQL-level functions (provenance_times, provenance_plus, etc.) to construct circuit gates. These calls route through the mmap worker to persist the circuit.

4. Each result tuple comes back with a UUID identifying the root gate of its provenance sub-circuit.

5. To evaluate provenance, the user calls functions like provenance_evaluate, probability_evaluate, or a compiled semiring evaluator (e.g., sr_boolean). These retrieve the circuit from mmap, build an in-memory GenericCircuit or BooleanCircuit, and traverse the DAG applying semiring operations.

The OID Cache: constants_t

PostgreSQL identifies types, functions, and operators by their Object Identifiers (OIDs). ProvSQL needs to reference its own types and functions when constructing rewritten query trees, so it caches their OIDs in a constants_t structure.

Key fields:

• Type OIDs: OID_TYPE_UUID, OID_TYPE_AGG_TOKEN, OID_TYPE_GATE_TYPE, and standard types (BOOL, INT, FLOAT, VARCHAR).

• Function OIDs: OID_FUNCTION_PROVENANCE_PLUS, OID_FUNCTION_PROVENANCE_TIMES, OID_FUNCTION_PROVENANCE_MONUS, OID_FUNCTION_PROVENANCE_DELTA, OID_FUNCTION_PROVENANCE_AGGREGATE, OID_FUNCTION_PROVENANCE_SEMIMOD, etc.

• Gate-type mapping: GATE_TYPE_TO_OID[nb_gate_types] maps each gate_type enum value to the OID of the corresponding provenance_gate enum member in PostgreSQL.

• Status flag: ok is true if the OIDs were loaded successfully (false if the extension is not installed in the current database).

The cache is populated by get_constants, which looks up OIDs in the system catalogs on first call and stores them per-database. Subsequent calls return the cached values without catalog access.

Gate Types

The provenance circuit is a directed acyclic graph (DAG) whose nodes are gates. Each gate has a type from the gate_type enum defined in provsql_utils.h:

Gate type	Meaning
gate_input	Leaf gate representing a base-table tuple.
gate_plus	Semiring addition (⊕): duplicate elimination, UNION.
gate_times	Semiring multiplication (⊗): joins, cross products.
gate_monus	M-semiring monus (⊖): EXCEPT.
gate_project	Projection gate (where-provenance).
gate_eq	Equijoin gate (where-provenance).
gate_zero	Semiring additive identity (0).
gate_one	Semiring multiplicative identity (1).
gate_agg	Aggregation operator.
gate_semimod	Semimodule scalar multiplication (for aggregation).
gate_delta	Delta operator (δ-semiring).
gate_value	Scalar constant value.
gate_mulinput	Multivalued input (for Boolean probability).
gate_update	Update-provenance gate.

Edges (wires) connect parent gates to their children, forming the provenance formula for each query result tuple.