# pg_trickle

[![Build](https://github.com/grove/pg-trickle/actions/workflows/build.yml/badge.svg)](https://github.com/grove/pg-trickle/actions/workflows/build.yml)
[![CI](https://github.com/grove/pg-trickle/actions/workflows/ci.yml/badge.svg)](https://github.com/grove/pg-trickle/actions/workflows/ci.yml)
[![Release](https://github.com/grove/pg-trickle/actions/workflows/release.yml/badge.svg)](https://github.com/grove/pg-trickle/actions/workflows/release.yml)
[![Coverage](https://codecov.io/gh/grove/pg-trickle/branch/main/graph/badge.svg)](https://codecov.io/gh/grove/pg-trickle)
[![Benchmarks](https://img.shields.io/badge/Benchmarks-view-blue)](docs/BENCHMARK.md)
[![Roadmap](https://img.shields.io/badge/Roadmap-view-informational)](ROADMAP.md)
[![License: Apache 2.0](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](LICENSE)
[![PostgreSQL 18](https://img.shields.io/badge/PostgreSQL-18-blue?logo=postgresql&logoColor=white)](https://www.postgresql.org/)
[![pgrx 0.17](https://img.shields.io/badge/pgrx-0.17-orange)](https://github.com/pgcentralfoundation/pgrx)

> **Early Release Notice:** This project is in an early stage of development and is **not yet production ready**. APIs, configuration options, and internal behavior may change without notice. Use at your own risk and please report any issues you encounter.

> For a plain-language description of the problem pg_trickle solves, the differential dataflow approach, and the hybrid CDC architecture, read **[ESSENCE.md](ESSENCE.md)**.

**Stream Tables for PostgreSQL 18**

pg_trickle brings declarative, automatically-refreshing materialized views to PostgreSQL, inspired by the [DBSP](https://arxiv.org/abs/2203.16684) differential dataflow framework ([comparison](docs/research/DBSP_COMPARISON.md)). Define a SQL query and a schedule bound (or cron schedule); the extension handles the rest.

## History and Motivation

This project started with a practical goal. We were inspired by existing data platforms built around pipelines that keep themselves incrementally up to date, and we wanted to bring that same style of self-maintaining data flow directly into PostgreSQL. In particular, we needed support for recursive CTEs, which were essential to the kinds of pipelines we had in mind. We could not find an open-source incremental view maintenance system that matched that requirement, so pg_trickle began as an attempt to close that gap.

It also became an experiment in what coding agents could realistically help build. We set out to develop pg_trickle without editing code by hand, while still holding it to the same bar we would expect from any other systems project: broad feature coverage, strong code quality, extensive tests, and thorough documentation. Skepticism toward AI-written software is reasonable; the right way to evaluate pg_trickle is by the codebase, the tests, and the docs.

That constraint changed how we worked. Agents can produce a lot of surface area quickly, but database systems are unforgiving of vague assumptions and hidden edge cases. To make the project hold together, we had to be unusually explicit about architecture, operator semantics, failure handling, and test coverage. In practice, that pushed us toward more written design, more reviewable behavior, and more verification than a quick prototype would normally get.

We also do not think the use of AI should lower the standard for trust. If anything, it raises it. The point of the experiment was not to ask people to trust the toolchain; it was to see whether disciplined use of coding agents could help produce a serious, inspectable PostgreSQL extension. Whether that worked is for readers and users to judge, but the intent is simple: make the code, the tests, the documentation, and the tradeoffs visible enough that the project can stand on its own merits.

## Key Features

- **Declarative** — define a query and a schedule bound (or cron expression); the extension schedules and executes refreshes automatically.
- **Four refresh modes** — `AUTO` (smart default: DIFFERENTIAL when possible, FULL fallback), `DIFFERENTIAL` (incremental delta), `FULL` (complete recomputation), and `IMMEDIATE` (synchronous in-transaction maintenance via statement-level triggers with transition tables).
- **Differential View Maintenance (DVM)** — only processes changed rows, not the entire base table. Delta queries are derived automatically from the defining query's operator tree.
- **Transactional IVM (IMMEDIATE mode)** — stream tables can be maintained **within the same transaction** as the base table DML, providing read-your-writes consistency. Supports all DVM operators including window functions, LATERAL joins, scalar subqueries, cascading IMMEDIATE stream tables, WITH RECURSIVE, and TopK micro-refresh.
- **CTE Support** — full support for Common Table Expressions. Non-recursive CTEs are inlined and differentiated algebraically. Multi-reference CTEs share delta computation. Recursive CTEs (`WITH RECURSIVE`) work in FULL, DIFFERENTIAL, and IMMEDIATE modes.
- **TopK support** — `ORDER BY ... LIMIT N [OFFSET M]` queries are accepted and refreshed correctly via scoped recomputation. Paged TopK (`OFFSET M`) supports server-side pagination.
- **ALTER QUERY** — change the defining query of an existing stream table online. The engine classifies schema changes (same / compatible / incompatible), migrates the storage table, updates the dependency graph, and runs a full refresh. Compatible changes preserve the storage table OID so views and publications remain valid.
- **Trigger-based CDC** — lightweight `AFTER` row-level triggers capture changes into buffer tables. No logical replication slots or `wal_level = logical` required. Triggers are created and dropped automatically.
- **Hybrid CDC (optional)** — when `wal_level = logical` is available, the system can automatically transition from triggers to WAL-based (logical replication) capture for lower write-side overhead. Controlled by the `pg_trickle.cdc_mode` GUC (`trigger` / `auto` / `wal`).
- **DAG-aware scheduling** — stream tables that depend on other stream tables are refreshed in topological order. `CALCULATED` schedule propagation is supported.
- **Diamond dependency consistency** — diamond-shaped DAGs (A→B→D, A→C→D) can be refreshed atomically to prevent split-version reads.
- **Circular dependency support** — monotone dependency cycles can be enabled explicitly and are refreshed to a fixed point with convergence guardrails and monitoring.
- **Watermark gating** — external loaders can publish per-source watermarks so downstream refreshes wait until related sources are aligned.
- **Multi-database auto-discovery** — a single launcher worker automatically spawns per-database scheduler workers for every database that has the extension installed. No manual per-database configuration needed.
- **Crash-safe** — advisory locks prevent concurrent refreshes; crash recovery marks in-flight refreshes as failed and resumes normal operation.
- **Observable** — built-in monitoring views, refresh history, slot health checks, staleness reporting, SCC status, watermark status, `NOTIFY`-based alerting, and dedicated helper functions such as `health_check`, `change_buffer_sizes`, `dependency_tree`, `refresh_timeline`, `trigger_inventory`, `list_sources`, `diamond_groups`, and `pgt_scc_status`.

## SQL Support

Every operator listed here works in `DIFFERENTIAL` mode (incremental delta computation) unless noted otherwise. `FULL` mode always works — it re-runs the entire query on each refresh.

| Category | Feature | Support | Notes |
|---|---|---|---|
| **Core** | Table scan | ✅ Full | |
| **Core** | Projection (`SELECT` expressions) | ✅ Full | |
| **Filtering** | `WHERE` clause | ✅ Full | |
| **Filtering** | `HAVING` clause | ✅ Full | Applied as a filter on top of the aggregate delta |
| **Joins** | `INNER JOIN` | ✅ Full | Equi-joins are optimal; non-equi-joins also work |
| **Joins** | `LEFT OUTER JOIN` | ✅ Full | |
| **Joins** | `RIGHT OUTER JOIN` | ✅ Full | Automatically converted to `LEFT JOIN` with swapped operands |
| **Joins** | `FULL OUTER JOIN` | ✅ Full | 8-part delta; may be slower than inner joins on high-churn data |
| **Joins** | `NATURAL JOIN` | ✅ Full | Resolved at parse time; explicit equi-join synthesized |
| **Joins** | Nested joins (3+ tables) | ✅ Full | `a JOIN b JOIN c` — snapshot subqueries with disambiguated columns |
| **Aggregation** | `GROUP BY` + `COUNT`, `SUM`, `AVG` | ✅ Full | Fully algebraic — no rescan needed |
| **Aggregation** | `GROUP BY` + `MIN`, `MAX` | ✅ Full | Semi-algebraic — uses `LEAST`/`GREATEST` merge; per-group rescan when extremum is deleted |
| **Aggregation** | `GROUP BY` + `BOOL_AND`, `BOOL_OR` | ✅ Full | Group-rescan — affected groups are re-aggregated from source |
| **Aggregation** | `GROUP BY` + `STRING_AGG`, `ARRAY_AGG` | ✅ Full | Group-rescan — affected groups are re-aggregated from source |
| **Aggregation** | `GROUP BY` + `JSON_AGG`, `JSONB_AGG` | ✅ Full | Group-rescan — affected groups are re-aggregated from source |
| **Aggregation** | `GROUP BY` + `BIT_AND`, `BIT_OR`, `BIT_XOR` | ✅ Full | Group-rescan — affected groups are re-aggregated from source |
| **Aggregation** | `GROUP BY` + `JSON_OBJECT_AGG`, `JSONB_OBJECT_AGG` | ✅ Full | Group-rescan — affected groups are re-aggregated from source |
| **Aggregation** | `GROUP BY` + `STDDEV`, `STDDEV_POP`, `STDDEV_SAMP` | ✅ Full | Group-rescan — affected groups are re-aggregated from source |
| **Aggregation** | `GROUP BY` + `VARIANCE`, `VAR_POP`, `VAR_SAMP` | ✅ Full | Group-rescan — affected groups are re-aggregated from source |
| **Aggregation** | `MODE() WITHIN GROUP (ORDER BY …)` | ✅ Full | Ordered-set aggregate — group-rescan strategy |
| **Aggregation** | `PERCENTILE_CONT` / `PERCENTILE_DISC` `WITHIN GROUP (ORDER BY …)` | ✅ Full | Ordered-set aggregate — group-rescan strategy |
| **Aggregation** | `CORR`, `COVAR_*`, `REGR_*` (12 regression aggregates) | ✅ Full | Group-rescan strategy |
| **Aggregation** | `JSON_ARRAYAGG`, `JSON_OBJECTAGG` (SQL standard) | ✅ Full | Group-rescan strategy; PostgreSQL 16+ |
| **Aggregation** | `FILTER (WHERE …)` on aggregates | ✅ Full | Filter predicate applied within delta computation |
| **Deduplication** | `DISTINCT` | ✅ Full | Reference-counted multiplicity tracking |
| **Deduplication** | `DISTINCT ON (…)` | ✅ Full | Auto-rewritten to `ROW_NUMBER()` window subquery |
| **Set operations** | `UNION ALL` | ✅ Full | |
| **Set operations** | `UNION` (deduplicated) | ✅ Full | Composed as `UNION ALL` + `DISTINCT` |
| **Set operations** | `INTERSECT` / `EXCEPT` | ✅ Full | Dual-count multiplicity tracking with LEAST / GREATEST boundary crossing |
| **Subqueries** | Subquery in `FROM` | ✅ Full | `(SELECT …) AS alias` |
| **Subqueries** | `EXISTS` / `NOT EXISTS` in `WHERE` | ✅ Full | Semi-join / anti-join delta operators |
| **Subqueries** | `IN` / `NOT IN` (subquery) in `WHERE` | ✅ Full | Rewritten to semi-join / anti-join |
| **Subqueries** | `ALL` (subquery) in `WHERE` | ✅ Full | Rewritten to anti-join via `NOT EXISTS` |
| **Subqueries** | Scalar subquery in `SELECT` | ✅ Full | `(SELECT max(x) FROM t)` in target list |
| **Subqueries** | Scalar subquery in `WHERE` | ✅ Full | Auto-rewritten to `CROSS JOIN` |
| **CTEs** | Non-recursive `WITH` | ✅ Full | Single & multi-reference; shared delta computation |
| **CTEs** | Recursive `WITH RECURSIVE` | ✅ Full | Both `FULL` and `DIFFERENTIAL` modes (semi-naive + DRed) |
| **Window functions** | `ROW_NUMBER`, `RANK`, `SUM OVER`, etc. | ✅ Full | Partition-based recomputation |
| **Window functions** | Window frame clauses | ✅ Full | `ROWS`, `RANGE`, `GROUPS` with `BETWEEN` bounds and `EXCLUDE` |
| **Window functions** | Named `WINDOW` clauses | ✅ Full | `WINDOW w AS (...)` resolved from query-level window definitions |
| **Window functions** | Multiple `PARTITION BY` clauses | ✅ Full | Same partition key used directly; different keys fall back to full recomputation |
| **LATERAL SRFs** | `jsonb_array_elements`, `unnest`, `jsonb_each`, etc. | ✅ Full | Row-scoped recomputation in DIFFERENTIAL mode |
| **JSON_TABLE** | `JSON_TABLE(expr, path COLUMNS (...))` | ✅ Full | PostgreSQL 17+; modeled as lateral function |
| **Expressions** | `CASE WHEN … THEN … ELSE … END` | ✅ Full | Both searched and simple CASE |
| **Expressions** | `COALESCE`, `NULLIF`, `GREATEST`, `LEAST` | ✅ Full | |
| **Expressions** | `IN (list)`, `BETWEEN`, `IS DISTINCT FROM` | ✅ Full | Including `NOT IN`, `NOT BETWEEN`, `IS NOT DISTINCT FROM` |
| **Expressions** | `IS TRUE/FALSE/UNKNOWN` | ✅ Full | All boolean test variants |
| **Expressions** | `SIMILAR TO`, `ANY(array)`, `ALL(array)` | ✅ Full | |
| **Expressions** | `ARRAY[…]`, `ROW(…)` | ✅ Full | |
| **Expressions** | `CURRENT_DATE`, `CURRENT_TIMESTAMP`, etc. | ✅ Full | All SQL value functions |
| **Expressions** | Array subscript, field access | ✅ Full | `arr[1]`, `(rec).field`, `(data).*` |
| **Grouping** | `GROUPING SETS` / `CUBE` / `ROLLUP` | ✅ Full | Auto-rewritten to `UNION ALL` of `GROUP BY` queries |
| **Safety** | Volatile function/operator detection | ✅ Full | VOLATILE expressions rejected in DIFFERENTIAL and IMMEDIATE; STABLE functions warned |
| **Source tables** | Tables without primary key | ✅ Full | Content-hash row identity via all columns |
| **Source tables** | Views as sources | ✅ Full | Auto-inlined as subqueries; CDC triggers land on base tables |
| **Source tables** | Materialized views | ❌ DIFF / ✅ FULL | Rejected in DIFFERENTIAL (stale snapshot); allowed in FULL |
| **TopK** | `ORDER BY ... LIMIT N [OFFSET M]` | ✅ Full | TopK stream tables store only N rows (optionally from position M+1); scoped recomputation via MERGE |
| **Ordering** | `ORDER BY` (without LIMIT) | ⚠️ Ignored | Accepted but silently discarded; storage row order is undefined |
| **Ordering** | `LIMIT` / `OFFSET` (without ORDER BY) | ❌ Rejected | Not supported — stream tables materialize the full result set |

See [docs/DVM_OPERATORS.md](docs/DVM_OPERATORS.md) for the full differentiation rules and CTE tiers.

## Quick Start

### Prerequisites

- PostgreSQL 18.x
- Rust 1.82+ with [pgrx](https://github.com/pgcentralfoundation/pgrx) 0.17.x

### Install

```bash
# Build and install the extension
cargo pgrx install --release --pg-config $(pg_config --bindir)/pg_config

# Or package for deployment
cargo pgrx package --pg-config $(pg_config --bindir)/pg_config
```

Add to `postgresql.conf`:

```ini
shared_preload_libraries = 'pg_trickle'
max_worker_processes = 8
```

> **Note:** `wal_level = logical` and `max_replication_slots` are **not** required by default. CDC uses lightweight row-level triggers unless you opt in to WAL-based capture via `pg_trickle.cdc_mode = 'auto'` (see [CONFIGURATION.md](docs/CONFIGURATION.md)).

Restart PostgreSQL, then:

```sql
CREATE EXTENSION pg_trickle;
```

### Kubernetes (CloudNativePG)

pg_trickle is distributed as a minimal OCI extension image for [CloudNativePG Image Volume Extensions](https://cloudnative-pg.io/docs/1.28/imagevolume_extensions/). The image is `scratch`-based (< 10 MB) and contains only the extension files — no PostgreSQL server, no OS.

```bash
docker pull ghcr.io/grove/pg_trickle-ext:0.2.2
```

Deploy with the official CNPG PostgreSQL 18 operand image:

```yaml
# In your Cluster resource
spec:
  imageName: ghcr.io/cloudnative-pg/postgresql:18
  postgresql:
    shared_preload_libraries: [pg_trickle]
    extensions:
      - name: pg-trickle
        image:
          reference: ghcr.io/grove/pg_trickle-ext:0.2.2
```

See [cnpg/cluster-example.yaml](cnpg/cluster-example.yaml) and [cnpg/database-example.yaml](cnpg/database-example.yaml) for complete examples. Requires Kubernetes 1.33+ and CNPG 1.28+.

### Usage

```sql
-- Create base tables
CREATE TABLE orders (
    id    INT PRIMARY KEY,
    region TEXT,
    amount NUMERIC
);

INSERT INTO orders VALUES
    (1, 'US', 100), (2, 'EU', 200),
    (3, 'US', 300), (4, 'APAC', 50);

-- Create a stream table — AUTO mode (default): DIFFERENTIAL when possible, FULL fallback.
-- Schedule defaults to 'calculated' (derived from consumer refresh cycles).
SELECT pgtrickle.create_stream_table(
    'regional_totals',
    'SELECT region, SUM(amount) AS total, COUNT(*) AS cnt
     FROM orders GROUP BY region'
);

-- Explicit IMMEDIATE mode: updated within the same transaction as DML
SELECT pgtrickle.create_stream_table(
    'regional_totals_live',
    'SELECT region, SUM(amount) AS total, COUNT(*) AS cnt
     FROM orders GROUP BY region',
    schedule     => NULL,
    refresh_mode => 'IMMEDIATE'
);

-- Explicit schedule and mode
SELECT pgtrickle.create_stream_table(
    'hourly_totals',
    'SELECT region, SUM(amount) AS total FROM orders GROUP BY region',
    schedule     => '@hourly',
    refresh_mode => 'FULL'
);

-- Query the stream table like any regular table
SELECT * FROM regional_totals;

-- Manual refresh (scheduler also refreshes automatically)
SELECT pgtrickle.refresh_stream_table('regional_totals');

-- Change the defining query online (ALTER QUERY)
SELECT pgtrickle.alter_stream_table(
    'regional_totals',
    query => 'SELECT region, SUM(amount) AS total, COUNT(*) AS cnt
              FROM orders WHERE active GROUP BY region'
);

-- Check status and overall health
SELECT * FROM pgtrickle.pgt_status();
SELECT * FROM pgtrickle.health_check();  -- OK/WARN/ERROR triage

-- View monitoring stats
SELECT * FROM pgtrickle.pg_stat_stream_tables;
SELECT * FROM pgtrickle.dependency_tree();  -- ASCII DAG view
SELECT * FROM pgtrickle.change_buffer_sizes();  -- CDC buffer health

-- Drop when no longer needed
SELECT pgtrickle.drop_stream_table('regional_totals');
```

## Documentation

| Document | Description |
|---|---|
| [GETTING_STARTED.md](docs/GETTING_STARTED.md) | Hands-on tutorial building an org-chart with stream tables |
| [INSTALL.md](INSTALL.md) | Detailed installation and configuration guide |
| [docs/SQL_REFERENCE.md](docs/SQL_REFERENCE.md) | Complete SQL function reference |
| [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) | System architecture and data flow |
| [docs/DVM_OPERATORS.md](docs/DVM_OPERATORS.md) | Supported operators and differentiation rules |
| [docs/CONFIGURATION.md](docs/CONFIGURATION.md) | GUC variables and tuning guide |
| [ROADMAP.md](ROADMAP.md) | Release milestones and future plans (current milestone: v0.8.0) |

### Research & Plans

| Document | Description |
|---|---|
| [plans/sql/PLAN_NATIVE_SYNTAX.md](plans/sql/PLAN_NATIVE_SYNTAX.md) | Plan for `CREATE MATERIALIZED VIEW ... WITH (pgtrickle.stream)` syntax |
| [plans/sql/REPORT_CUSTOM_SQL_SYNTAX.md](plans/sql/REPORT_CUSTOM_SQL_SYNTAX.md) | Research: PostgreSQL extension syntax mechanisms |

### Deep-Dive Tutorials

| Tutorial | Description |
|---|---|
| [What Happens on INSERT](docs/tutorials/WHAT_HAPPENS_ON_INSERT.md) | Full 7-phase lifecycle of a single INSERT through the pipeline |
| [What Happens on UPDATE](docs/tutorials/WHAT_HAPPENS_ON_UPDATE.md) | D+I split, group key changes, net-effect for multiple UPDATEs |
| [What Happens on DELETE](docs/tutorials/WHAT_HAPPENS_ON_DELETE.md) | Reference counting, group deletion, INSERT+DELETE cancellation |
| [What Happens on TRUNCATE](docs/tutorials/WHAT_HAPPENS_ON_TRUNCATE.md) | Why TRUNCATE bypasses triggers and recovery strategies |

## Known Limitations

The following SQL features are **rejected with clear error messages** explaining the limitation and suggesting rewrites. See [FAQ — Why Are These SQL Features Not Supported?](docs/FAQ.md#why-are-these-sql-features-not-supported) for detailed technical explanations.

| Feature | Reason | Suggested Rewrite |
|---|---|---|
| Materialized views in DIFFERENTIAL | CDC triggers cannot track `REFRESH MATERIALIZED VIEW` | Use the underlying query directly, or use FULL mode |
| `TABLESAMPLE` | Stream tables materialize the complete result set; sampling at define-time is not meaningful | Use `WHERE random() < fraction` in the consuming query |
| Window functions in expressions | Window functions inside `CASE`, `COALESCE`, arithmetic, etc. cannot be differentially maintained | Move window function to a separate column |
| `LIMIT` / `OFFSET` without `ORDER BY` | Stream tables materialize the full result set; arbitrary row subsets are non-deterministic | Use `ORDER BY ... LIMIT N` for TopK, or apply LIMIT when querying the stream table |
| `FOR UPDATE` / `FOR SHARE` | Row-level locking not applicable | Remove the locking clause |

### Stream Table Restrictions

Stream tables are regular PostgreSQL heap tables, but their contents are managed exclusively by the refresh engine. See [FAQ — Why Are These Stream Table Operations Restricted?](docs/FAQ.md#why-are-these-stream-table-operations-restricted) for detailed explanations.

| Operation | Allowed? | Notes |
|---|---|---|
| ST references other STs | ✅ Yes | DAG-ordered refresh; monotone cycles supported via `pg_trickle.allow_circular` |
| Views reference STs | ✅ Yes | Standard PostgreSQL views work normally |
| Materialized views reference STs | ✅ Yes | Requires separate `REFRESH MATERIALIZED VIEW` |
| Logical replication of STs | ✅ Yes | `__pgt_row_id` column is replicated; subscribers receive materialized data only |
| Direct DML on STs | ❌ No | Contents managed by the refresh engine |
| Foreign keys on STs | ❌ No | Bulk `MERGE` during refresh does not respect FK ordering |
| User triggers on STs | ✅ Supported | Supported in DIFFERENTIAL mode; suppressed during FULL refresh (see `pg_trickle.user_triggers` GUC) |

See [SQL Reference — Restrictions & Interoperability](docs/SQL_REFERENCE.md#restrictions--interoperability) for details and examples.

## How It Works

### DIFFERENTIAL / FULL Mode (scheduled)

1. **Create** — `pgtrickle.create_stream_table()` parses the defining query into an operator tree, creates a storage table, installs lightweight CDC triggers on source tables, and registers the ST in the catalog.

2. **Capture** — Changes to base tables are captured via the hybrid CDC layer. By default, `AFTER INSERT/UPDATE/DELETE` row-level triggers write to per-source change buffer tables in the `pgtrickle_changes` schema. With `pg_trickle.cdc_mode = 'auto'`, the system transitions to WAL-based capture (logical replication) after the first successful refresh for lower write-side overhead.

3. **Schedule** — A background worker wakes periodically (default: 1s) and checks which STs have exceeded their schedule (or whose cron schedule has fired). STs are scheduled for refresh in topological order.

4. **Differentiate** — The DVM engine differentiates the defining query's operator tree to produce a delta query (ΔQ) that computes only the changes since the last refresh.

5. **Apply** — The delta query is executed and its results (INSERT/DELETE actions with row IDs) are merged into the storage table.

6. **Version** — Each refresh records a frontier (per-source LSN positions) and a data timestamp, implementing a simplified Data Versioning System (DVS).

### IMMEDIATE Mode (transactional)

When `refresh_mode = 'IMMEDIATE'`, the stream table is maintained **within the same transaction** as the base table DML:

1. **BEFORE triggers** acquire an advisory lock on the stream table.
2. **AFTER triggers** (with `REFERENCING NEW TABLE AS ... OLD TABLE AS ...`) copy transition tables to temp tables.
3. The DVM engine computes the delta from the transition tables (via `DeltaSource::TransitionTable`).
4. The delta is applied to the stream table via DELETE + INSERT ON CONFLICT.

No change buffer tables, no scheduler, no WAL infrastructure. The stream table is always up-to-date within the current transaction.

Supported SQL features include all DIFFERENTIAL-mode constructs: JOINs, aggregates, window functions, LATERAL, scalar subqueries, `WITH RECURSIVE`, TopK, and more. `WITH RECURSIVE` in IMMEDIATE mode uses semi-naive evaluation (INSERT-only) or Delete-and-Rederive (DELETE/UPDATE), bounded by `pg_trickle.ivm_recursive_max_depth` (default 100). TopK stream tables in IMMEDIATE mode use micro-refresh (recompute top-K on every DML), gated by `pg_trickle.ivm_topk_max_limit`.

## Architecture

```
┌─────────────────────────────────────────────┐
│              PostgreSQL 18                  │
│                                             │
│  ┌─────────┐    ┌──────────┐   ┌─────────┐  │
│  │  Source │    │  Stream  │   │pg_trickle│  │
│  │  Tables │───▸│  Tables  │   │ Catalog │  │
│  └────┬────┘    └──────────┘   └─────────┘  │
│       │                                     │
│  ┌────▼─────────────────────────────┐       │
│  │     Hybrid CDC Layer              │       │
│  │     Triggers (default) or WAL     │       │
│  └────┬─────────────────────────────┘       │
│       │                                     │
│  ┌────▼─────────────────────────────┐       │
│  │     Change Buffer Tables         │       │
│  │     pgtrickle_changes schema      │       │
│  └────┬─────────────────────────────┘       │
│       │                                     │
│  ┌────▼─────────────────────────────┐       │
│  │     DVM Engine                   │       │
│  │  ┌────────┐ ┌─────────────────┐  │       │
│  │  │ Parser │ │ Differentiation │  │       │
│  │  │ OpTree │ │  Delta Query ΔQ │  │       │
│  │  └────────┘ └─────────────────┘  │       │
│  └────┬─────────────────────────────┘       │
│       │                                     │
│  ┌────▼─────────────────────────────┐       │
│  │     Refresh Executor             │       │
│  │  Full / Differential / Immediate │       │
│  └────┬─────────────────────────────┘       │
│       │                                     │
│  ┌────▼─────────────────────────────┐       │
│  │     Background Scheduler         │       │
│  │     DAG-aware, schedule-based    │       │
│  └──────────────────────────────────┘       │
└─────────────────────────────────────────────┘
```

## Testing

```bash
# Unit tests (no database required)
cargo test --lib

# Integration tests (requires Docker for Testcontainers)
cargo test --test '*'

# Property-based tests
cargo test --test property_tests

# All tests
cargo test

# Benchmarks
cargo bench
```

**Test counts:** ~1,042 unit tests + integration tests + 819 E2E tests.

### Code Coverage

Generate coverage reports for unit tests using [`cargo-llvm-cov`](https://github.com/taiki-e/cargo-llvm-cov):

```bash
# Full report (HTML + LCOV)
just coverage

# Or use the script directly
./scripts/coverage.sh

# LCOV only (for CI / Codecov upload)
./scripts/coverage.sh --lcov

# Quick terminal summary
./scripts/coverage.sh --text
```

Reports are written to `coverage/`:
- `coverage/html/index.html` — browsable HTML report
- `coverage/lcov.info` — LCOV data for upload to [Codecov](https://codecov.io)

Coverage is automatically collected and uploaded to Codecov on every push to `main` and on pull requests via the **Coverage** GitHub Actions workflow.

## Contributors

- [Geir O. Grønmo](https://github.com/grove)
- [Baard H. Rehn Johansen](https://github.com/BaardBouvet)
- [GitHub Copilot](https://github.com/features/copilot) (AI pair programmer)

## License

[Apache License 2.0](LICENSE)
