Caching

LIRS Buffer Pool

The buffer pool keeps frequently accessed disk pages in memory. Absolute DB uses the LIRS (Low Inter-reference Recency Set) algorithm, which outperforms LRU by distinguishing between frequently and infrequently referenced pages.

Property	Value
Algorithm	LIRS (patent-free)
Page states	LIR (hot), rHIR (resident cold), nHIR (evicted cold)
All operations	O(1) amortised
Default size	64 KB (minimal), configurable via `--buffer-pool-mb`
HIR ratio	1–5% of capacity (tunable)

bash

# Set buffer pool to 4 GB
absdb-server --buffer-pool-mb 4096

# Check hit rate
SELECT * FROM absdb_stats WHERE name = 'buffer_pool_hit_rate';

Prepared Statement Plan Cache

Parsed and optimised query plans are cached to avoid re-parsing identical queries:

Property	Value
Capacity	1,024 plans
Key	64-bit hash of normalised SQL text
Eviction	LRU
Invalidation	Automatic on DDL (table alter/drop/create)
Latency reduction	~85 µs → ~2 µs for cached plans

sql

-- Use prepared statements for plan caching
PREPARE get_user AS SELECT * FROM users WHERE id = $1;
EXECUTE get_user(42);
EXECUTE get_user(99);  -- uses cached plan

-- Deallocate when done
DEALLOCATE get_user;

Query Result Cache

Entire query results can be cached with configurable TTL:

Property	Value
Capacity	1,024 entries, 256 MB total
Max per result	1 MB
Key	64-bit hash of SQL text + parameters
TTL	Configurable per query (default: disabled)
Invalidation	Automatic on any write to dependent tables
Thread safety	Lock-free concurrent reads, exclusive writes

sql

-- Cache this query result for 60 seconds
SELECT /*+CACHE(ttl=60)*/ region, SUM(revenue)
FROM sales
GROUP BY region;

-- Next identical query within 60s returns cached result instantly
-- Any INSERT/UPDATE/DELETE on 'sales' invalidates the cache entry

Cache Statistics

sql

-- Buffer pool stats
SELECT * FROM absdb_stats WHERE name LIKE 'buffer_pool%';

-- Plan cache stats
SELECT * FROM absdb_stats WHERE name LIKE 'plan_cache%';

-- Result cache stats
SELECT * FROM absdb_stats WHERE name LIKE 'qcache%';

LIRS Buffer Pool

Prepared Statement Plan Cache

Query Result Cache

Cache Statistics

Related Documentation

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