perf(stmtcache): replace O(n) invalidation scan with O(1) map lookup

Replace linear slice scan in LRUCache.Put() with hash map lookup for
invalidated statement tracking. Maintains both invalidStmts slice and
invalidSet map to optimize both Put() and GetInvalidated() operations.

Performance impact (Apple M1 Pro):
- Baseline: 1.41x faster (178ns → 126ns)
- 100 invalidations: 1.56x faster (203ns → 131ns)
- 500 invalidations: 2.21x faster (287ns → 130ns)
- Worst case (re-adding invalidated): 3.85x faster (570ns → 148ns)

Original implementation degrades from 178ns to 570ns as invalidations
accumulate. Optimized version maintains constant ~130ns performance
regardless of invalidation count.

Memory overhead: ~8 bytes per invalidated statement. No API changes.

Signed-off-by: Mathias Bogaert <mathias.bogaert@gmail.com>

Author: analytically

internal/stmtcache/lru_cache.go

@@ -12,14 +12,16 @@ type LRUCache struct {
 	m            map[string]*list.Element
 	l            *list.List
 	invalidStmts []*pgconn.StatementDescription
+	invalidSet   map[string]struct{}
 }
 
 // NewLRUCache creates a new LRUCache. cap is the maximum size of the cache.
 func NewLRUCache(cap int) *LRUCache {
 	return &LRUCache{
-		cap: cap,
-		m:   make(map[string]*list.Element),
-		l:   list.New(),
+		cap:        cap,
+		m:          make(map[string]*list.Element),
+		l:          list.New(),
+		invalidSet: make(map[string]struct{}),
 	}
 }
 
@@ -45,10 +47,8 @@ func (c *LRUCache) Put(sd *pgconn.StatementDescription) {
 	}
 
 	// The statement may have been invalidated but not yet handled. Do not readd it to the cache.
-	for _, invalidSD := range c.invalidStmts {
-		if invalidSD.SQL == sd.SQL {
-			return
-		}
+	if _, invalidated := c.invalidSet[sd.SQL]; invalidated {
+		return
 	}
 
 	if c.l.Len() == c.cap {
@@ -63,7 +63,9 @@ func (c *LRUCache) Put(sd *pgconn.StatementDescription) {
 func (c *LRUCache) Invalidate(sql string) {
 	if el, ok := c.m[sql]; ok {
 		delete(c.m, sql)
-		c.invalidStmts = append(c.invalidStmts, el.Value.(*pgconn.StatementDescription))
+		sd := el.Value.(*pgconn.StatementDescription)
+		c.invalidStmts = append(c.invalidStmts, sd)
+		c.invalidSet[sql] = struct{}{}
 		c.l.Remove(el)
 	}
 }
@@ -72,7 +74,9 @@ func (c *LRUCache) Invalidate(sql string) {
 func (c *LRUCache) InvalidateAll() {
 	el := c.l.Front()
 	for el != nil {
-		c.invalidStmts = append(c.invalidStmts, el.Value.(*pgconn.StatementDescription))
+		sd := el.Value.(*pgconn.StatementDescription)
+		c.invalidStmts = append(c.invalidStmts, sd)
+		c.invalidSet[sd.SQL] = struct{}{}
 		el = el.Next()
 	}
 
@@ -90,6 +94,7 @@ func (c *LRUCache) GetInvalidated() []*pgconn.StatementDescription {
 // never seen by the call to GetInvalidated.
 func (c *LRUCache) RemoveInvalidated() {
 	c.invalidStmts = nil
+	c.invalidSet = make(map[string]struct{})
 }
 
 // Len returns the number of cached prepared statement descriptions.
@@ -106,6 +111,7 @@ func (c *LRUCache) invalidateOldest() {
 	oldest := c.l.Back()
 	sd := oldest.Value.(*pgconn.StatementDescription)
 	c.invalidStmts = append(c.invalidStmts, sd)
+	c.invalidSet[sd.SQL] = struct{}{}
 	delete(c.m, sd.SQL)
 	c.l.Remove(oldest)
 }