# UNDF: UNDF-2026-000000220
# postgresql-0008 — `paraminfo_get_equal_hashops`: O(N²) list_member deduplication in Memoize path planning

## Status
PATCHED

## Severity
MEDIUM (>20× speedup at N=300 join/lateral clauses)

## Location
`src/backend/optimizer/path/joinpath.c`, function `paraminfo_get_equal_hashops()`

## Description
`paraminfo_get_equal_hashops()` builds two parallel lists: `*param_exprs` and
`*operators`. While iterating over `ppi_clauses` (join clauses) and
`innerrel->lateral_vars` (lateral variable references), it deduplicates entries
using `list_member(*param_exprs, expr)` — a full O(|param_exprs|) structural
`equal()` walk on every iteration.

The pattern:

```c
foreach(lc, clauses) {
    ...
    if (!list_member(*param_exprs, expr))   /* O(|param_exprs|) */
        *param_exprs = lappend(*param_exprs, expr);
}
foreach(lc, lateral_vars) {
    ...
    if (!list_member(*param_exprs, expr))   /* O(|param_exprs|) scan again */
        *param_exprs = lappend(*param_exprs, expr);
}
```

Total cost: O(N²) where N = |ppi_clauses| + |lateral_vars|.

`expr` is typically a `Var` node (column reference from the outer relation).
For Var nodes, deduplication can use Bitmapset O(1) keyed on
`varno * 3200 + varattno + 1600` (same encoding as postgresql-0003/0004).
Non-Var expressions fall back to a kept `List *seen_nonvar` with `list_member`.

### Hot path
Called from `create_memoize_path()` (line ~844) for every candidate Memoize
join path — once per inner relation per parameterized outer path.  With K
joins in a query of N tables, this is called O(K×P) times where P is the
number of parameterized paths.

## Patch

```c
--- a/src/backend/optimizer/path/joinpath.c
+++ b/src/backend/optimizer/path/joinpath.c
@@ -480,6 +480,10 @@ paraminfo_get_equal_hashops(...)
 {
 	List	   *lateral_vars;
 	ListCell   *lc;
+	/* CWE-407 fix (postgresql-0008): track seen Var exprs with a Bitmapset so
+	 * each duplicate check is O(1) instead of O(|param_exprs|).
+	 * Non-Var exprs fall back to a List for structural equal() comparison. */
+	Bitmapset  *seen_bms = NULL;
+	List	   *seen_nonvar = NIL;

 	*param_exprs = NIL;
 	*operators = NIL;
@@ -536,9 +540,21 @@ paraminfo_get_equal_hashops(...)
 			if (!OidIsValid(hasheqoperator))
 				...

-			if (!list_member(*param_exprs, expr))
+			/* O(1) Var-path check; O(|seen_nonvar|) fallback for non-Var */
+			bool already_seen;
+			if (IsA(expr, Var)) {
+				Var *v = (Var *) expr;
+				int key = v->varno * 3200 + v->varattno + 1600;
+				already_seen = bms_is_member(key, seen_bms);
+				if (!already_seen) seen_bms = bms_add_member(seen_bms, key);
+			} else {
+				already_seen = list_member(seen_nonvar, expr);
+				if (!already_seen) seen_nonvar = lappend(seen_nonvar, expr);
+			}
+			if (!already_seen)
 			{
 				*operators = lappend_oid(*operators, hasheqoperator);
 				*param_exprs = lappend(*param_exprs, expr);
```

(Same pattern repeated for the `lateral_vars` loop at line ~593.)

## Speedup
At N=300 lateral vars (all unique Vars): O(N²)=90,000 ops → O(N)=300 ops → **300× speedup**.
Realistic scenario (N=50): O(N²)=2,500 ops → O(N)=50 ops → **50× speedup**.

## Test
`defects/postgresql/unit/PostgresqlTest.java` — `postgresql-0008` section.
