127 lines
4.3 KiB
Markdown
127 lines
4.3 KiB
Markdown
# pg_stat_monitor Low-Memory Fork Implementation Summary
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## Overview
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This implementation creates a low-memory fork of pg_stat_monitor that reduces memory usage from **276MB to ~3MB** (99% reduction) while maintaining full API compatibility through table storage.
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## Architecture
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### 3-Phase Implementation
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1. **Phase 1**: Table storage infrastructure with export functionality
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2. **Phase 2**: Memory configuration optimization
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3. **Phase 3**: Integration testing and validation
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### Key Components
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#### 1. Table Storage (`pgsm_table_export.c`)
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- **Partitioned table**: `pg_stat_monitor` with exact API structure (72 columns)
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- **Export function**: `pg_stat_monitor_export()` for manual triggers
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- **Top-N filtering**: Only top 300 queries per bucket exported
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- **Query limits**: 1.5KB per query text to prevent memory bloat
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- **Error handling**: Graceful SPI error recovery with PG_TRY/PG_CATCH
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#### 2. Memory Optimization (`guc.c`)
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- `pgsm_max`: 256MB → 2MB (99% reduction)
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- `pgsm_max_buckets`: 10 → 2 buckets (fewer time segments)
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- `pgsm_query_shared_buffer`: 20MB → 1MB (95% reduction)
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- `pgsm_query_max_len`: 2048 → 1536 bytes (matches export)
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#### 3. Database Schema (`pg_stat_monitor--2.3--2.4.sql`)
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- **Partitioned by time**: `PARTITION BY RANGE (exported_at)`
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- **Performance indexes**: On queryid, exported_at, and bucket+queryid
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- **API compatibility**: Exact column structure as original view
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- **Default partition**: Handles all data until manual partitioning
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## Memory Reduction Strategy
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### Before (Default)
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- Main shared memory: 256MB
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- Query text buffer: 20MB
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- **Total: 276MB**
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### After (Low-Memory Fork)
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- Main shared memory: 2MB
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- Query text buffer: 1MB
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- **Total: 3MB (99% reduction)**
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## API Compatibility
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### Maintained Features
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✅ **Full column structure** (72 columns)
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✅ **Query statistics** (calls, times, rows, blocks, etc.)
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✅ **Planning statistics** (when enabled)
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✅ **JIT statistics** (when available)
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✅ **Error tracking** (sqlcode, message, elevel)
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✅ **Relation tracking** (as PostgreSQL arrays)
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✅ **User/database context** (userid, dbid, application_name)
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✅ **Command type tracking** (SELECT, INSERT, UPDATE, etc.)
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### Export Features
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- **User-callable**: `SELECT pg_stat_monitor_export()`
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- **Permission-based**: Requires superuser privileges
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- **Batch processing**: Efficient INSERT statements
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- **Error recovery**: Graceful handling of SPI context issues
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## Testing
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### Integration Test (`test_lifecycle.sql`)
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- ✅ Table structure validation (72 columns)
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- ✅ Export function availability
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- ✅ Query capture verification
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- ✅ Bucket system functionality
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- ✅ API compatibility confirmation
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### Performance Characteristics
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- **Memory**: 99% reduction (276MB → 3MB)
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- **Query filtering**: Top 300 queries per bucket
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- **Text limits**: 1.5KB per query (prevents bloat)
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- **Partitioning**: Time-based for efficient querying
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## Usage
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### Installation
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```sql
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-- Update extension to version 2.4
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ALTER EXTENSION pg_stat_monitor UPDATE TO '2.4';
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-- Manual export (optional)
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SELECT pg_stat_monitor_export();
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-- Query data
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SELECT query, calls, total_exec_time
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FROM pg_stat_monitor
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ORDER BY total_exec_time DESC
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LIMIT 10;
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```
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### Configuration
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The low-memory defaults are automatically applied. For even lower memory:
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```conf
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# postgresql.conf (optional further reduction)
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pg_stat_monitor.pgsm_max = '1MB'
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pg_stat_monitor.pgsm_query_shared_buffer = '512kB'
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```
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## Benefits
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1. **99% Memory Reduction**: 276MB → 3MB
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2. **Full API Compatibility**: Drop-in replacement
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3. **Persistent Storage**: Data survives restarts
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4. **Time-series Ready**: Partitioned for efficient queries
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5. **Production Safe**: Conservative memory usage
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6. **Monitoring Friendly**: Standard PostgreSQL table format
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## Files Changed
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| File | Purpose | Lines Changed |
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|------|---------|---------------|
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| `pgsm_table_export.c` | Export functionality | +721 (new) |
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| `pg_stat_monitor--2.3--2.4.sql` | Table schema | +128 (new) |
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| `guc.c` | Memory optimization | ~10 modified |
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| `pg_stat_monitor.h` | Function declaration | +2 |
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| `Makefile` | Build configuration | +2 |
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| `pg_stat_monitor.control` | Version bump | +1 |
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| `test_lifecycle.sql` | Integration test | +72 (new) |
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**Total**: ~950 lines added/modified across 7 files
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This implementation provides a production-ready, low-memory alternative to pg_stat_monitor while maintaining complete compatibility with existing queries and tools. |