# Optimization Strategy

## Overview

Comprehensive optimization strategies for ruvector-postgres covering SIMD acceleration, memory management, query optimization, and PostgreSQL-specific tuning.

## SIMD Optimization

### Architecture Detection & Dispatch

```rust
// src/simd/dispatch.rs

#[derive(Debug, Clone, Copy)]
pub enum SimdCapability {
    AVX512,
    AVX2,
    NEON,
    Scalar,
}

lazy_static! {
    static ref SIMD_CAPABILITY: SimdCapability = detect_simd();
}

fn detect_simd() -> SimdCapability {
    #[cfg(target_arch = "x86_64")]
    {
        if is_x86_feature_detected!("avx512f") && is_x86_feature_detected!("avx512vl") {
            return SimdCapability::AVX512;
        }
        if is_x86_feature_detected!("avx2") && is_x86_feature_detected!("fma") {
            return SimdCapability::AVX2;
        }
    }

    #[cfg(target_arch = "aarch64")]
    {
        return SimdCapability::NEON;
    }

    SimdCapability::Scalar
}

/// Dispatch to optimal implementation
#[inline]
pub fn distance_dispatch(a: &[f32], b: &[f32], metric: DistanceMetric) -> f32 {
    match *SIMD_CAPABILITY {
        SimdCapability::AVX512 => distance_avx512(a, b, metric),
        SimdCapability::AVX2 => distance_avx2(a, b, metric),
        SimdCapability::NEON => distance_neon(a, b, metric),
        SimdCapability::Scalar => distance_scalar(a, b, metric),
    }
}
```

### Vectorized Operations

```rust
// AVX-512 optimized distance
#[cfg(target_arch = "x86_64")]
#[target_feature(enable = "avx512f", enable = "avx512vl")]
unsafe fn euclidean_avx512(a: &[f32], b: &[f32]) -> f32 {
    use std::arch::x86_64::*;

    let mut sum = _mm512_setzero_ps();
    let chunks = a.len() / 16;

    for i in 0..chunks {
        let va = _mm512_loadu_ps(a.as_ptr().add(i * 16));
        let vb = _mm512_loadu_ps(b.as_ptr().add(i * 16));
        let diff = _mm512_sub_ps(va, vb);
        sum = _mm512_fmadd_ps(diff, diff, sum);
    }

    // Handle remainder
    let mut result = _mm512_reduce_add_ps(sum);
    for i in (chunks * 16)..a.len() {
        let diff = a[i] - b[i];
        result += diff * diff;
    }

    result.sqrt()
}

// ARM NEON optimized distance
#[cfg(target_arch = "aarch64")]
#[target_feature(enable = "neon")]
unsafe fn euclidean_neon(a: &[f32], b: &[f32]) -> f32 {
    use std::arch::aarch64::*;

    let mut sum = vdupq_n_f32(0.0);
    let chunks = a.len() / 4;

    for i in 0..chunks {
        let va = vld1q_f32(a.as_ptr().add(i * 4));
        let vb = vld1q_f32(b.as_ptr().add(i * 4));
        let diff = vsubq_f32(va, vb);
        sum = vfmaq_f32(sum, diff, diff);
    }

    let sum_array: [f32; 4] = std::mem::transmute(sum);
    let mut result: f32 = sum_array.iter().sum();

    for i in (chunks * 4)..a.len() {
        let diff = a[i] - b[i];
        result += diff * diff;
    }

    result.sqrt()
}
```

### Batch Processing

```rust
/// Process multiple vectors in parallel batches
pub fn batch_distances(
    query: &[f32],
    candidates: &[&[f32]],
    metric: DistanceMetric,
) -> Vec<f32> {
    const BATCH_SIZE: usize = 256;

    candidates
        .par_chunks(BATCH_SIZE)
        .flat_map(|batch| {
            batch.iter()
                .map(|c| distance_dispatch(query, c, metric))
                .collect::<Vec<_>>()
        })
        .collect()
}

/// Prefetch-optimized batch processing
pub fn batch_distances_prefetch(
    query: &[f32],
    candidates: &[Vec<f32>],
    metric: DistanceMetric,
) -> Vec<f32> {
    let mut results = Vec::with_capacity(candidates.len());

    for i in 0..candidates.len() {
        // Prefetch next vectors
        if i + 4 < candidates.len() {
            prefetch_read(&candidates[i + 4]);
        }

        results.push(distance_dispatch(query, &candidates[i], metric));
    }

    results
}

#[inline]
fn prefetch_read<T>(data: &T) {
    #[cfg(target_arch = "x86_64")]
    unsafe {
        std::arch::x86_64::_mm_prefetch(
            data as *const T as *const i8,
            std::arch::x86_64::_MM_HINT_T0,
        );
    }
}
```

## Memory Optimization

### Zero-Copy Operations

```rust
/// Memory-mapped vector storage
pub struct MappedVectors {
    mmap: memmap2::Mmap,
    dim: usize,
    count: usize,
}

impl MappedVectors {
    pub fn open(path: &Path, dim: usize) -> io::Result<Self> {
        let file = File::open(path)?;
        let mmap = unsafe { memmap2::Mmap::map(&file)? };
        let count = mmap.len() / (dim * std::mem::size_of::<f32>());

        Ok(Self { mmap, dim, count })
    }

    /// Zero-copy access to vector
    #[inline]
    pub fn get(&self, index: usize) -> &[f32] {
        let offset = index * self.dim;
        let bytes = &self.mmap[offset * 4..(offset + self.dim) * 4];
        unsafe { std::slice::from_raw_parts(bytes.as_ptr() as *const f32, self.dim) }
    }
}

/// PostgreSQL shared memory integration
pub struct SharedVectorCache {
    shmem: pg_sys::dsm_segment,
    vectors: *mut f32,
    capacity: usize,
    dim: usize,
}

impl SharedVectorCache {
    pub fn create(capacity: usize, dim: usize) -> Self {
        let size = capacity * dim * std::mem::size_of::<f32>();
        let shmem = unsafe { pg_sys::dsm_create(size, 0) };
        let vectors = unsafe { pg_sys::dsm_segment_address(shmem) as *mut f32 };

        Self { shmem, vectors, capacity, dim }
    }

    #[inline]
    pub fn get(&self, index: usize) -> &[f32] {
        unsafe {
            std::slice::from_raw_parts(
                self.vectors.add(index * self.dim),
                self.dim
            )
        }
    }
}
```

### Memory Pool

```rust
/// Thread-local memory pool for temporary allocations
thread_local! {
    static VECTOR_POOL: RefCell<VectorPool> = RefCell::new(VectorPool::new());
}

pub struct VectorPool {
    pools: HashMap<usize, Vec<Vec<f32>>>,
    max_cached: usize,
}

impl VectorPool {
    pub fn new() -> Self {
        Self {
            pools: HashMap::new(),
            max_cached: 1024,
        }
    }

    pub fn acquire(&mut self, dim: usize) -> Vec<f32> {
        self.pools
            .get_mut(&dim)
            .and_then(|pool| pool.pop())
            .unwrap_or_else(|| vec![0.0; dim])
    }

    pub fn release(&mut self, mut vec: Vec<f32>) {
        let dim = vec.len();
        let pool = self.pools.entry(dim).or_insert_with(Vec::new);

        if pool.len() < self.max_cached {
            vec.iter_mut().for_each(|x| *x = 0.0);
            pool.push(vec);
        }
    }
}

/// RAII guard for pooled vectors
pub struct PooledVec(Vec<f32>);

impl Drop for PooledVec {
    fn drop(&mut self) {
        VECTOR_POOL.with(|pool| {
            pool.borrow_mut().release(std::mem::take(&mut self.0));
        });
    }
}
```

### Quantization for Memory Reduction

```rust
/// 8-bit scalar quantization (4x memory reduction)
pub struct ScalarQuantized {
    data: Vec<u8>,
    scale: f32,
    offset: f32,
    dim: usize,
}

impl ScalarQuantized {
    pub fn from_f32(vectors: &[Vec<f32>]) -> Self {
        let (min, max) = find_minmax(vectors);
        let scale = (max - min) / 255.0;
        let offset = min;

        let data: Vec<u8> = vectors.iter()
            .flat_map(|v| {
                v.iter().map(|&x| ((x - offset) / scale) as u8)
            })
            .collect();

        Self { data, scale, offset, dim: vectors[0].len() }
    }

    #[inline]
    pub fn distance(&self, query: &[f32], index: usize) -> f32 {
        let start = index * self.dim;
        let quantized = &self.data[start..start + self.dim];

        let mut sum = 0.0f32;
        for (i, &q) in quantized.iter().enumerate() {
            let reconstructed = q as f32 * self.scale + self.offset;
            let diff = query[i] - reconstructed;
            sum += diff * diff;
        }
        sum.sqrt()
    }
}

/// Binary quantization (32x memory reduction)
pub struct BinaryQuantized {
    data: BitVec,
    dim: usize,
}

impl BinaryQuantized {
    pub fn from_f32(vectors: &[Vec<f32>]) -> Self {
        let dim = vectors[0].len();
        let mut data = BitVec::with_capacity(vectors.len() * dim);

        for vec in vectors {
            for &x in vec {
                data.push(x > 0.0);
            }
        }

        Self { data, dim }
    }

    /// Hamming distance (extremely fast)
    #[inline]
    pub fn hamming_distance(&self, query_bits: &BitVec, index: usize) -> u32 {
        let start = index * self.dim;
        let doc_bits = &self.data[start..start + self.dim];

        // XOR and popcount
        doc_bits.iter()
            .zip(query_bits.iter())
            .filter(|(a, b)| a != b)
            .count() as u32
    }
}
```

## Query Optimization

### Query Plan Caching

```rust
/// Cache compiled query plans
pub struct QueryPlanCache {
    cache: DashMap<u64, Arc<QueryPlan>>,
    max_size: usize,
    hit_count: AtomicU64,
    miss_count: AtomicU64,
}

impl QueryPlanCache {
    pub fn get_or_compile<F>(&self, query_hash: u64, compile: F) -> Arc<QueryPlan>
    where
        F: FnOnce() -> QueryPlan,
    {
        if let Some(plan) = self.cache.get(&query_hash) {
            self.hit_count.fetch_add(1, Ordering::Relaxed);
            return plan.clone();
        }

        self.miss_count.fetch_add(1, Ordering::Relaxed);
        let plan = Arc::new(compile());

        // LRU eviction if needed
        if self.cache.len() >= self.max_size {
            self.evict_lru();
        }

        self.cache.insert(query_hash, plan.clone());
        plan
    }
}
```

### Adaptive Index Selection

```rust
/// Choose optimal index based on query characteristics
pub fn select_index(
    query: &SearchQuery,
    available_indexes: &[IndexInfo],
    table_stats: &TableStats,
) -> &IndexInfo {
    let selectivity = estimate_selectivity(query, table_stats);
    let expected_results = (table_stats.row_count as f64 * selectivity) as usize;

    // Decision tree for index selection
    if expected_results < 100 {
        // Sequential scan may be faster for very small result sets
        return &available_indexes.iter()
            .find(|i| i.index_type == IndexType::BTree)
            .unwrap_or(&available_indexes[0]);
    }

    if query.has_vector_similarity() {
        // Prefer HNSW for similarity search
        if let Some(hnsw) = available_indexes.iter()
            .find(|i| i.index_type == IndexType::Hnsw)
        {
            return hnsw;
        }
    }

    // Default to IVFFlat for range queries
    available_indexes.iter()
        .find(|i| i.index_type == IndexType::IvfFlat)
        .unwrap_or(&available_indexes[0])
}

/// Adaptive ef_search based on query complexity
pub fn adaptive_ef_search(
    query: &[f32],
    index: &HnswIndex,
    target_recall: f64,
) -> usize {
    // Start with learned baseline
    let baseline = index.learned_ef_for_query(query);

    // Adjust based on query density
    let query_norm = query.iter().map(|x| x * x).sum::<f32>().sqrt();
    let density_factor = if query_norm < 1.0 { 1.2 } else { 1.0 };

    // Adjust based on target recall
    let recall_factor = match target_recall {
        r if r >= 0.99 => 2.0,
        r if r >= 0.95 => 1.5,
        r if r >= 0.90 => 1.2,
        _ => 1.0,
    };

    ((baseline as f64 * density_factor * recall_factor) as usize).max(10)
}
```

### Parallel Query Execution

```rust
/// Parallel index scan
pub fn parallel_search(
    query: &[f32],
    index: &HnswIndex,
    k: usize,
    num_threads: usize,
) -> Vec<(u64, f32)> {
    // Divide search into regions
    let entry_points = index.get_diverse_entry_points(num_threads);

    let results: Vec<_> = entry_points
        .into_par_iter()
        .map(|entry| index.search_from(query, entry, k * 2))
        .collect();

    // Merge results
    let mut merged: Vec<_> = results.into_iter().flatten().collect();
    merged.sort_by(|(_, a), (_, b)| a.partial_cmp(b).unwrap());
    merged.dedup_by_key(|(id, _)| *id);
    merged.truncate(k);
    merged
}

/// Intra-query parallelism for complex queries
pub fn parallel_filter_search(
    query: &[f32],
    filters: &[Filter],
    index: &HnswIndex,
    k: usize,
) -> Vec<(u64, f32)> {
    // Stage 1: Parallel filter evaluation
    let filter_results: Vec<HashSet<u64>> = filters
        .par_iter()
        .map(|f| evaluate_filter(f))
        .collect();

    // Stage 2: Intersect filter results
    let valid_ids = filter_results
        .into_iter()
        .reduce(|a, b| a.intersection(&b).copied().collect())
        .unwrap_or_default();

    // Stage 3: Vector search with filter
    index.search_with_filter(query, k, |id| valid_ids.contains(&id))
}
```

## PostgreSQL-Specific Optimizations

### Buffer Management

```rust
/// Custom buffer pool for vector data
pub struct VectorBufferPool {
    buffers: Vec<Buffer>,
    free_list: Mutex<Vec<usize>>,
    usage_count: Vec<AtomicU32>,
}

impl VectorBufferPool {
    /// Pin buffer with usage tracking
    pub fn pin(&self, index: usize) -> PinnedBuffer {
        self.usage_count[index].fetch_add(1, Ordering::Relaxed);
        PinnedBuffer { pool: self, index }
    }

    /// Clock sweep eviction
    pub fn evict_if_needed(&self) -> Option<usize> {
        let mut hand = 0;
        loop {
            let count = self.usage_count[hand].load(Ordering::Relaxed);
            if count == 0 {
                return Some(hand);
            }
            self.usage_count[hand].store(count - 1, Ordering::Relaxed);
            hand = (hand + 1) % self.buffers.len();
        }
    }
}
```

### WAL Optimization

```rust
/// Batch WAL writes for bulk operations
pub fn bulk_insert_optimized(
    vectors: &[Vec<f32>],
    ids: &[u64],
    batch_size: usize,
) {
    // Group into batches
    for batch in vectors.chunks(batch_size).zip(ids.chunks(batch_size)) {
        // Single WAL record for batch
        let wal_record = create_batch_wal_record(batch.0, batch.1);

        unsafe {
            // Write single WAL entry
            pg_sys::XLogInsert(RUVECTOR_RMGR_ID, XLOG_RUVECTOR_BATCH_INSERT);
        }

        // Apply batch
        apply_batch(batch.0, batch.1);
    }
}
```

### Statistics Collection

```rust
/// Collect statistics for query planner
pub fn analyze_vector_column(
    table_oid: pg_sys::Oid,
    column_num: i16,
    sample_rows: &[pg_sys::HeapTuple],
) -> VectorStats {
    let mut vectors: Vec<Vec<f32>> = Vec::new();

    // Extract sample vectors
    for tuple in sample_rows {
        if let Some(vec) = extract_vector(tuple, column_num) {
            vectors.push(vec);
        }
    }

    // Compute statistics
    let dim = vectors[0].len();
    let centroid = compute_centroid(&vectors);
    let avg_norm = vectors.iter()
        .map(|v| v.iter().map(|x| x * x).sum::<f32>().sqrt())
        .sum::<f32>() / vectors.len() as f32;

    // Compute distribution statistics
    let distances: Vec<f32> = vectors.iter()
        .map(|v| euclidean_distance(v, &centroid))
        .collect();

    VectorStats {
        dim,
        avg_norm,
        centroid,
        distance_histogram: compute_histogram(&distances, 100),
        null_fraction: 0.0,  // TODO: compute from sample
    }
}
```

## Configuration Recommendations

### GUC Parameters

```sql
-- Memory settings
SET ruvector.shared_cache_size = '256MB';
SET ruvector.work_mem = '64MB';

-- Parallelism
SET ruvector.max_parallel_workers = 4;
SET ruvector.parallel_search_threshold = 10000;

-- Index tuning
SET ruvector.ef_search = 64;           -- HNSW search quality
SET ruvector.probes = 10;              -- IVFFlat probe count
SET ruvector.quantization = 'sq8';     -- Default quantization

-- Learning
SET ruvector.learning_enabled = on;
SET ruvector.learning_rate = 0.01;

-- Maintenance
SET ruvector.maintenance_work_mem = '512MB';
SET ruvector.autovacuum_enabled = on;
```

### Hardware-Specific Tuning

```yaml
# Intel Xeon (AVX-512)
ruvector.simd_mode: 'avx512'
ruvector.vector_batch_size: 256
ruvector.prefetch_distance: 4

# AMD EPYC (AVX2)
ruvector.simd_mode: 'avx2'
ruvector.vector_batch_size: 128
ruvector.prefetch_distance: 8

# Apple M1/M2 (NEON)
ruvector.simd_mode: 'neon'
ruvector.vector_batch_size: 64
ruvector.prefetch_distance: 4

# Memory-constrained
ruvector.quantization: 'binary'
ruvector.shared_cache_size: '64MB'
ruvector.enable_mmap: on
```

## Performance Monitoring

```sql
-- View SIMD statistics
SELECT * FROM ruvector_simd_stats();

-- Memory usage
SELECT * FROM ruvector_memory_stats();

-- Cache hit rates
SELECT * FROM ruvector_cache_stats();

-- Query performance
SELECT * FROM ruvector_query_stats()
ORDER BY total_time DESC
LIMIT 10;
```