---
title: Materialized views
description: Pre-built ClickHouse materialized views for common analytics patterns
---

The pg_stat_ch ClickHouse schema includes four materialized views that aggregate `events_raw` data automatically. All views are populated by ClickHouse as events arrive -- no manual refresh is needed.

## Overview

| View | Purpose | Granularity | Retention |
|---|---|---|---|
| `events_recent_1h` | Real-time debugging | Per event | 1 hour TTL |
| `query_stats_5m` | Query performance dashboards | 5-minute buckets | Unbounded |
| `db_app_user_1m` | Load by application/user | 1-minute buckets | Unbounded |
| `errors_recent` | Error investigation | Per error event | 7-day TTL |

The typical workflow is: use materialized views to find patterns, then drill into `events_raw` for specific events.

## `events_recent_1h`

A copy of `events_raw` with a 1-hour TTL. ClickHouse automatically deletes events older than 1 hour from this table.

**Use cases:**
- "What queries ran in the last 5 minutes?"
- Real-time dashboards with sub-second refresh
- Quick debugging of ongoing issues

```sql
SELECT
    ts_start,
    db,
    cmd_type,
    duration_us / 1000 AS ms,
    rows,
    substring(query, 1, 100) AS query_preview
FROM pg_stat_ch.events_recent_1h
WHERE ts_start > now() - INTERVAL 5 MINUTE
ORDER BY ts_start DESC
LIMIT 50;
```

Because this table has a 1-hour TTL, queries against it are fast even if `events_raw` has weeks of data.

## `query_stats_5m`

Pre-aggregated query statistics in 5-minute buckets. This is the primary view for dashboards and performance monitoring.

**Use cases:**
- QPS trends over time
- Latency percentiles (p95/p99) per query
- Identifying slow queries and regressions
- Capacity planning

### Schema

| Column | Type | Description |
|---|---|---|
| `bucket` | `DateTime` | 5-minute bucket start time |
| `db` | `LowCardinality(String)` | Database name |
| `query_id` | `Int64` | Normalized query identifier |
| `cmd_type` | `LowCardinality(String)` | Command type |
| `calls_state` | `AggregateFunction(count)` | Call count |
| `duration_sum_state` | `AggregateFunction(sum, UInt64)` | Total duration (microseconds) |
| `duration_min_state` | `AggregateFunction(min, UInt64)` | Minimum duration |
| `duration_max_state` | `AggregateFunction(max, UInt64)` | Maximum duration |
| `duration_q_state` | `AggregateFunction(quantilesTDigest(0.95, 0.99), UInt64)` | Latency percentile digest |
| `rows_sum_state` | `AggregateFunction(sum, UInt64)` | Total rows |
| `shared_hit_sum_state` | `AggregateFunction(sum, Int64)` | Total buffer hits |
| `shared_read_sum_state` | `AggregateFunction(sum, Int64)` | Total buffer reads |

### Querying aggregate states

This view uses ClickHouse's `-State` / `-Merge` pattern. Columns store intermediate aggregate states that must be finalized with the corresponding `-Merge` function:

| To get | Use |
|---|---|
| Call count | `countMerge(calls_state)` |
| Total duration | `sumMerge(duration_sum_state)` |
| Min duration | `minMerge(duration_min_state)` |
| Max duration | `maxMerge(duration_max_state)` |
| p95 and p99 | `quantilesTDigestMerge(0.95, 0.99)(duration_q_state)` |
| Total rows | `sumMerge(rows_sum_state)` |
| Buffer hits | `sumMerge(shared_hit_sum_state)` |
| Buffer reads | `sumMerge(shared_read_sum_state)` |

This pattern allows correct re-aggregation across multiple 5-minute buckets. For example, a 1-hour p99 is computed correctly from 12 five-minute digests, rather than being an average of 12 p99 values.

### Top queries by p99 latency

```sql
SELECT
    query_id,
    cmd_type,
    countMerge(calls_state) AS calls,
    round(sumMerge(duration_sum_state) / countMerge(calls_state) / 1000, 2) AS avg_ms,
    round(quantilesTDigestMerge(0.95, 0.99)(duration_q_state)[1] / 1000, 2) AS p95_ms,
    round(quantilesTDigestMerge(0.95, 0.99)(duration_q_state)[2] / 1000, 2) AS p99_ms
FROM pg_stat_ch.query_stats_5m
WHERE bucket >= now() - INTERVAL 1 HOUR
GROUP BY query_id, cmd_type
ORDER BY p99_ms DESC
LIMIT 10;
```

### QPS over time

Each bucket is 5 minutes (300 seconds), so divide by 300 for per-second rate:

```sql
SELECT
    bucket,
    countMerge(calls_state) / 300 AS qps
FROM pg_stat_ch.query_stats_5m
WHERE bucket >= now() - INTERVAL 24 HOUR
GROUP BY bucket
ORDER BY bucket;
```

### Cache hit ratio trend

```sql
SELECT
    bucket,
    round(100 * sumMerge(shared_hit_sum_state) /
        (sumMerge(shared_hit_sum_state) + sumMerge(shared_read_sum_state) + 1), 2) AS hit_ratio_pct
FROM pg_stat_ch.query_stats_5m
WHERE bucket >= now() - INTERVAL 24 HOUR
GROUP BY bucket
ORDER BY bucket;
```

## `db_app_user_1m`

Load breakdown by database, application, user, and command type in 1-minute buckets.

**Use cases:**
- Which application is generating the most load?
- Per-tenant or per-user resource tracking
- Error rates by application
- Identifying misbehaving clients

### Schema

| Column | Type | Description |
|---|---|---|
| `bucket` | `DateTime` | 1-minute bucket start time |
| `db` | `LowCardinality(String)` | Database name |
| `app` | `LowCardinality(String)` | Application name |
| `username` | `LowCardinality(String)` | PostgreSQL username |
| `cmd_type` | `LowCardinality(String)` | Command type |
| `calls_state` | `AggregateFunction(count)` | Query count |
| `duration_sum_state` | `AggregateFunction(sum, UInt64)` | Total duration (microseconds) |
| `duration_q_state` | `AggregateFunction(quantilesTDigest(0.95, 0.99), UInt64)` | Latency percentile digest |
| `errors_sum_state` | `AggregateFunction(sum, UInt64)` | Error count |

### Load by application

```sql
SELECT
    app,
    countMerge(calls_state) AS total_queries,
    round(sumMerge(duration_sum_state) / 1000000, 2) AS total_seconds,
    round(quantilesTDigestMerge(0.95, 0.99)(duration_q_state)[2] / 1000, 2) AS p99_ms,
    sumMerge(errors_sum_state) AS errors
FROM pg_stat_ch.db_app_user_1m
WHERE bucket >= now() - INTERVAL 24 HOUR
GROUP BY app
ORDER BY total_seconds DESC;
```

### Error rate by database and user

```sql
SELECT
    db,
    username,
    countMerge(calls_state) AS queries,
    sumMerge(errors_sum_state) AS errors,
    round(100 * sumMerge(errors_sum_state) / countMerge(calls_state), 2) AS error_pct
FROM pg_stat_ch.db_app_user_1m
WHERE bucket >= now() - INTERVAL 1 HOUR
GROUP BY db, username
HAVING errors > 0
ORDER BY error_pct DESC;
```

## `errors_recent`

Recent error events with a 7-day TTL. This view filters `events_raw` to only rows where `err_elevel > 0`.

**Use cases:**
- "What errors happened in the last hour?"
- Incident investigation with query context
- Error rate alerting
- Identifying recurring error patterns by SQLSTATE

### Schema

This view stores the full event row (not aggregated):

`ts_start`, `db`, `username`, `app`, `client_addr`, `pid`, `query_id`, `err_sqlstate`, `err_elevel`, `err_message`, `query`

### Recent errors with context

```sql
SELECT
    ts_start,
    db,
    username,
    app,
    err_sqlstate,
    err_message,
    substring(query, 1, 200) AS query_preview
FROM pg_stat_ch.errors_recent
WHERE ts_start > now() - INTERVAL 1 HOUR
ORDER BY ts_start DESC
LIMIT 100;
```

### Error breakdown by SQLSTATE

```sql
SELECT
    err_sqlstate,
    count() AS occurrences,
    uniq(query_id) AS unique_queries,
    any(err_message) AS sample_message
FROM pg_stat_ch.errors_recent
WHERE ts_start > now() - INTERVAL 24 HOUR
GROUP BY err_sqlstate
ORDER BY occurrences DESC;
```

## Custom views

You can create your own materialized views on top of `events_raw` for project-specific analytics. For example, a view that tracks queries by table name or a view that computes per-minute error rates for alerting.

See the [ClickHouse documentation on materialized views](https://clickhouse.com/docs/en/guides/developer/cascading-materialized-views) for details on creating and managing views.
