1 files changed, 160 insertions, 0 deletions
diff --git a/sqlglot/optimizer/eliminate_joins.py b/sqlglot/optimizer/eliminate_joins.py
new file mode 100644
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+++ b/sqlglot/optimizer/eliminate_joins.py
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+from sqlglot import expressions as exp
+from sqlglot.optimizer.normalize import normalized
+from sqlglot.optimizer.scope import Scope, traverse_scope
+from sqlglot.optimizer.simplify import simplify
+
+
+def eliminate_joins(expression):
+    """
+    Remove unused joins from an expression.
+
+    This only removes joins when we know that the join condition doesn't produce duplicate rows.
+
+    Example:
+        >>> import sqlglot
+        >>> sql = "SELECT x.a FROM x LEFT JOIN (SELECT DISTINCT y.b FROM y) AS y ON x.b = y.b"
+        >>> expression = sqlglot.parse_one(sql)
+        >>> eliminate_joins(expression).sql()
+        'SELECT x.a FROM x'
+
+    Args:
+        expression (sqlglot.Expression): expression to optimize
+    Returns:
+        sqlglot.Expression: optimized expression
+    """
+    for scope in traverse_scope(expression):
+        # If any columns in this scope aren't qualified, it's hard to determine if a join isn't used.
+        # It's probably possible to infer this from the outputs of derived tables.
+        # But for now, let's just skip this rule.
+        if scope.unqualified_columns:
+            continue
+
+        joins = scope.expression.args.get("joins", [])
+
+        # Reverse the joins so we can remove chains of unused joins
+        for join in reversed(joins):
+            alias = join.this.alias_or_name
+            if _should_eliminate_join(scope, join, alias):
+                join.pop()
+                scope.remove_source(alias)
+    return expression
+
+
+def _should_eliminate_join(scope, join, alias):
+    inner_source = scope.sources.get(alias)
+    return (
+        isinstance(inner_source, Scope)
+        and not _join_is_used(scope, join, alias)
+        and (
+            (join.side == "LEFT" and _is_joined_on_all_unique_outputs(inner_source, join))
+            or (not join.args.get("on") and _has_single_output_row(inner_source))
+        )
+    )
+
+
+def _join_is_used(scope, join, alias):
+    # We need to find all columns that reference this join.
+    # But columns in the ON clause shouldn't count.
+    on = join.args.get("on")
+    if on:
+        on_clause_columns = set(id(column) for column in on.find_all(exp.Column))
+    else:
+        on_clause_columns = set()
+    return any(column for column in scope.source_columns(alias) if id(column) not in on_clause_columns)
+
+
+def _is_joined_on_all_unique_outputs(scope, join):
+    unique_outputs = _unique_outputs(scope)
+    if not unique_outputs:
+        return False
+
+    _, join_keys, _ = join_condition(join)
+    remaining_unique_outputs = unique_outputs - set(c.name for c in join_keys)
+    return not remaining_unique_outputs
+
+
+def _unique_outputs(scope):
+    """Determine output columns of `scope` that must have a unique combination per row"""
+    if scope.expression.args.get("distinct"):
+        return set(scope.expression.named_selects)
+
+    group = scope.expression.args.get("group")
+    if group:
+        grouped_expressions = set(group.expressions)
+        grouped_outputs = set()
+
+        unique_outputs = set()
+        for select in scope.selects:
+            output = select.unalias()
+            if output in grouped_expressions:
+                grouped_outputs.add(output)
+                unique_outputs.add(select.alias_or_name)
+
+        # All the grouped expressions must be in the output
+        if not grouped_expressions.difference(grouped_outputs):
+            return unique_outputs
+        else:
+            return set()
+
+    if _has_single_output_row(scope):
+        return set(scope.expression.named_selects)
+
+    return set()
+
+
+def _has_single_output_row(scope):
+    return isinstance(scope.expression, exp.Select) and (
+        all(isinstance(e.unalias(), exp.AggFunc) for e in scope.selects)
+        or _is_limit_1(scope)
+        or not scope.expression.args.get("from")
+    )
+
+
+def _is_limit_1(scope):
+    limit = scope.expression.args.get("limit")
+    return limit and limit.expression.this == "1"
+
+
+def join_condition(join):
+    """
+    Extract the join condition from a join expression.
+
+    Args:
+        join (exp.Join)
+    Returns:
+        tuple[list[str], list[str], exp.Expression]:
+            Tuple of (source key, join key, remaining predicate)
+    """
+    name = join.this.alias_or_name
+    on = join.args.get("on") or exp.TRUE
+    on = on.copy()
+    source_key = []
+    join_key = []
+
+    # find the join keys
+    # SELECT
+    # FROM x
+    # JOIN y
+    #   ON x.a = y.b AND y.b > 1
+    #
+    # should pull y.b as the join key and x.a as the source key
+    if normalized(on):
+        for condition in on.flatten() if isinstance(on, exp.And) else [on]:
+            if isinstance(condition, exp.EQ):
+                left, right = condition.unnest_operands()
+                left_tables = exp.column_table_names(left)
+                right_tables = exp.column_table_names(right)
+
+                if name in left_tables and name not in right_tables:
+                    join_key.append(left)
+                    source_key.append(right)
+                    condition.replace(exp.TRUE)
+                elif name in right_tables and name not in left_tables:
+                    join_key.append(right)
+                    source_key.append(left)
+                    condition.replace(exp.TRUE)
+
+    on = simplify(on)
+    remaining_condition = None if on == exp.TRUE else on
+
+    return source_key, join_key, remaining_condition