# v0.59.0 — Performance & Observability (Full Details)

> **Summary:** [v0.59.0.md](v0.59.0.md)
> **Assessment source:** [plans/PLAN_OVERALL_ASSESSMENT_12.md](../plans/PLAN_OVERALL_ASSESSMENT_12.md)
> **Findings addressed:** P-1, P-2, P-3, P-4, P-5, P-6, P-7, O-1, O-2, O-3, O-4, O-5, O-6

---

## Performance Improvements

### PERF-1: Batched Monitor Buffer-Growth Check

**Assessment finding:** P-1 (HIGH)
**File:** `src/monitor/health.rs`

**Before:** The per-ST health check loop called `SELECT count(*)::bigint FROM
pgtrickle_changes.changes_{oid}` for each CDC-enabled source, producing
O(N × monitor_ticks) SPI round-trips per second.  On a cluster with 100
sources at 10 Hz, this was ~1 000 SPI calls/sec.

**After:** A single SPI query fans out across all CDC-enabled sources using a
`VALUES` lateral union and a subquery per buffer table.  The query returns one
row per source with its pending-row count.  The monitor loop then applies the
threshold test in Rust without further SPI calls.

Approximate overhead reduction: from ~100 SPI calls per health-check cycle to
1 SPI call.

### PERF-2: Query Hash Cached on Catalog Row

**Assessment finding:** P-2 (MEDIUM)
**Files:** `src/refresh/merge/mod.rs`, `src/catalog.rs`,
`sql/pg_trickle--{prev}--0.59.0.sql`

A new `defining_query_hash BIGINT NOT NULL DEFAULT 0` column is added to
`pgtrickle.pgt_stream_tables`.  The hash is computed during `CREATE STREAM
TABLE` / `ALTER STREAM TABLE QUERY` and stored persistently.  The per-refresh
`DefaultHasher` computation over the full query string is removed.

The SQL upgrade script (`sql/pg_trickle--0.58.0--0.59.0.sql`) adds the column
and backfills it for all existing rows:
```sql
ALTER TABLE pgtrickle.pgt_stream_tables ADD COLUMN IF NOT EXISTS
    defining_query_hash BIGINT NOT NULL DEFAULT 0;
UPDATE pgtrickle.pgt_stream_tables
    SET defining_query_hash = hashtextextended(defining_query, 0)::bigint;
```

### PERF-3: Arc<str> Merge Template Strings

**Assessment finding:** P-3 (MEDIUM)
**File:** `src/refresh/merge/mod.rs`

`CachedMergeTemplate` holds eight SQL template strings.  Under a 1 000
refresh/s workload, cloning the struct on each cache hit produced ~60 MB/s of
string allocation.

The eight template fields are changed from `String` to `Arc<str>`.  A clone
of the struct becomes O(1) (8 atomic reference-count increments) regardless
of template size.

### PERF-4: Single MERGE_TEMPLATE_CACHE Borrow

**Assessment finding:** P-4 (MEDIUM)
**File:** `src/refresh/merge/mod.rs`

Two separate `MERGE_TEMPLATE_CACHE.with(|cache| { cache.borrow().get(…) })`
calls — one for `cached_non_monotonic`, one for `cached_is_deduplicated` —
were each marking the same LRU entry as recently used.  They are merged into
a single borrow that extracts both flags atomically:
```rust
let (non_monotonic, is_deduplicated) = MERGE_TEMPLATE_CACHE.with(|cache| {
    cache.borrow().peek(&st.pgt_id)
        .map(|e| (e.is_all_algebraic, e.is_deduplicated))
        .unwrap_or((false, false))
});
```
`peek()` is used to avoid spurious LRU promotion during read-only inspection.

### PERF-5: WAL Decoder UPDATE Vec Pre-allocation

**Assessment finding:** P-5 (MEDIUM)
**File:** `src/wal_decoder.rs`

The UPDATE handler previously allocated 5 separate `Vec<String>` objects per
row (column names for DELETE part, column names for INSERT part, placeholder
lists, parameter arrays) plus multiple `to_string()` calls per column.

After the fix:
- A single `Vec::with_capacity(num_columns)` is allocated for column names at
  the top of the function.
- Placeholder strings are written into a pre-allocated `String` with
  `write!(buf, "${}",  n)`.
- The separate D+I parameter vectors share a `Vec::with_capacity(2 * num_columns)`
  allocation.

Estimated reduction: 5× fewer heap allocations per UPDATE row.

### PERF-6: Frontier Borrow in Source-Change Probe

**Assessment finding:** P-6 (MEDIUM)
**File:** `src/scheduler/mod.rs`

`has_stream_table_source_changes()` previously called
`st.frontier.clone().unwrap_or_default()`, producing a full `Frontier`
clone (a `HashMap<i64, FrontierEntry>`) for every upstream-dependency probe.

**After:**
```rust
let frontier = st.frontier.as_ref().unwrap_or(&Frontier::EMPTY);
```
A new `Frontier::EMPTY` static provides a zero-allocation default.

### PERF-7: Diamond Detection Lazy Intersection

**Assessment finding:** P-7 (LOW)
**File:** `src/dag.rs`

Branch ancestor intersection now uses `HashSet::intersection()` as a lazy
iterator that breaks on the first shared node, avoiding the `collect()` into
a temporary `Vec` for every branch pair.  Branches with no shared ancestors
(the common case) exit without any allocation.

---

## Observability Improvements

### OBS-1: CDC Lag Percentile Metrics

**Assessment finding:** O-1 (MEDIUM)
**Files:** `src/shmem.rs`, `src/monitor/mod.rs`, `monitoring/prometheus/pg_trickle_queries.yml`

A rolling 15-minute reservoir sampler is added to shared memory, recording the
age (in seconds) of each frontier advancement.  The sampler uses a 256-slot
fixed-size circular buffer per worker that is reduced to percentiles on metric
scrape.

New Prometheus gauges:
- `pg_trickle_cdc_lag_p50_seconds`
- `pg_trickle_cdc_lag_p95_seconds`
- `pg_trickle_cdc_lag_p99_seconds`

### OBS-2: Parallel Worker Utilisation Metrics

**Assessment finding:** O-2 (MEDIUM)
**Files:** `src/scheduler/pool.rs`, `src/shmem.rs`

Two new shared-memory atomics:
- `PARALLEL_QUEUE_DEPTH` (`PgAtomic<u64>`) — incremented when a job is
  enqueued, decremented when a worker picks it up.
- `WORKER_IDLE_MS_TOTAL` (`PgAtomic<u64>`) — cumulative milliseconds
  workers have spent in the idle wait loop.

Both are exposed at the `/metrics` endpoint.

### OBS-3: WAL Decoder Pending-Record Metric

**Assessment finding:** O-3 (MEDIUM)
**File:** `src/wal_decoder.rs`

After each poll cycle, the WAL decoder writes the number of buffered-but-not-yet-
written records to a new `WAL_DECODER_PENDING_RECORDS` shmem atomic.  The
Prometheus endpoint reads and resets this counter on each scrape, exposing
it as `pg_trickle_wal_decoder_pending_records`.

Alerting recommendation added to `monitoring/prometheus/pg_trickle_alerts.yml`:
fire if `pg_trickle_wal_decoder_pending_records > 10000` for 5 minutes.

### OBS-4: Refresh-Mode Ratio Counters

**Assessment finding:** O-4 (MEDIUM)
**Files:** `src/refresh/orchestrator.rs`, `src/shmem.rs`

Two counter arrays (one per stream table, keyed by `pgt_id`) track the
cumulative count of DIFFERENTIAL and FULL refresh cycles.  Both are exposed
at the `/metrics` endpoint with a `pgt_id` label.  The
`postgres_exporter`-compatible queries block in
`monitoring/prometheus/pg_trickle_queries.yml` is updated.

### OBS-5: application_name in Background Workers

**Assessment finding:** O-5 (MEDIUM)
**File:** `src/scheduler/mod.rs`, `src/scheduler/scheduler_loop.rs`,
`src/scheduler/pool.rs`, `src/scheduler/dispatch.rs`, `src/wal_decoder.rs`

Each background-worker role now executes:
```sql
SET application_name = 'pg_trickle_<role>'
```
immediately after its first `Spi::connect()`.  Role names:
- `pg_trickle_launcher` — database discovery loop
- `pg_trickle_scheduler` — per-database scheduler
- `pg_trickle_pool_N` — parallel refresh workers (N = worker index)
- `pg_trickle_dispatcher` — parallel refresh DAG dispatcher
- `pg_trickle_wal_decoder` — WAL / logical replication poller

### OBS-6: Backup & Restore Documentation

**Assessment finding:** O-6 (LOW)
**File:** `INSTALL.md`

New "Backup & Restore" section added at the end of the installation guide.
Key content:

```bash
# Include both schemas when dumping a pg_trickle-enabled database:
pg_dump --schema=public --schema=pgtrickle --schema=pgtrickle_changes \
        mydb > mydb_backup.sql

# After restore, validate catalog integrity:
psql mydb -c "SELECT * FROM pgtrickle.health_check();"
```

The section also notes that change-buffer tables (`pgtrickle_changes.*`) are
per-OID and that OID assignment may differ after restore, so `pg_trickle_repair_stream_table()`
should be run on each stream table immediately after restore.

---

## Upgrade Notes

The SQL upgrade script `sql/pg_trickle--0.58.0--0.59.0.sql` adds one column
to `pgtrickle.pgt_stream_tables`:
```sql
ALTER TABLE pgtrickle.pgt_stream_tables
    ADD COLUMN IF NOT EXISTS defining_query_hash BIGINT NOT NULL DEFAULT 0;
```
The column is backfilled automatically.  No existing stream tables need to be
recreated.
