# sqlglot.optimizer.merge_subqueries API documentation

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# sqlglot.optimizer.merge_subqueries

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      1from __future__ import annotations
      2
      3import typing as t
      4
      5from collections import defaultdict
      6
      7from sqlglot import expressions as exp
      8from sqlglot.helper import find_new_name
      9from sqlglot.optimizer.scope import Scope, traverse_scope
     10
     11if t.TYPE_CHECKING:
     12    from sqlglot._typing import E
     13
     14    FromOrJoin = t.Union[exp.From, exp.Join]
     15
     16
     17def merge_subqueries(expression: E, leave_tables_isolated: bool = False) -> E:
     18    """
     19    Rewrite sqlglot AST to merge derived tables into the outer query.
     20
     21    This also merges CTEs if they are selected from only once.
     22
     23    Example:
     24        >>> import sqlglot
     25        >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT x.a FROM x) CROSS JOIN y")
     26        >>> merge_subqueries(expression).sql()
     27        'SELECT x.a FROM x CROSS JOIN y'
     28
     29    If `leave_tables_isolated` is True, this will not merge inner queries into outer
     30    queries if it would result in multiple table selects in a single query:
     31        >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT x.a FROM x) CROSS JOIN y")
     32        >>> merge_subqueries(expression, leave_tables_isolated=True).sql()
     33        'SELECT a FROM (SELECT x.a FROM x) CROSS JOIN y'
     34
     35    Inspired by https://dev.mysql.com/doc/refman/8.0/en/derived-table-optimization.html
     36
     37    Args:
     38        expression (sqlglot.Expression): expression to optimize
     39        leave_tables_isolated (bool):
     40    Returns:
     41        sqlglot.Expression: optimized expression
     42    """
     43    expression = merge_ctes(expression, leave_tables_isolated)
     44    expression = merge_derived_tables(expression, leave_tables_isolated)
     45    return expression
     46
     47
     48# If a derived table has these Select args, it can't be merged
     49UNMERGABLE_ARGS = set(exp.Select.arg_types) - {
     50    "expressions",
     51    "from",
     52    "joins",
     53    "where",
     54    "order",
     55    "hint",
     56}
     57
     58
     59# Projections in the outer query that are instances of these types can be replaced
     60# without getting wrapped in parentheses, because the precedence won't be altered.
     61SAFE_TO_REPLACE_UNWRAPPED = (
     62    exp.Column,
     63    exp.EQ,
     64    exp.Func,
     65    exp.NEQ,
     66    exp.Paren,
     67)
     68
     69
     70def merge_ctes(expression: E, leave_tables_isolated: bool = False) -> E:
     71    scopes = traverse_scope(expression)
     72
     73    # All places where we select from CTEs.
     74    # We key on the CTE scope so we can detect CTES that are selected from multiple times.
     75    cte_selections = defaultdict(list)
     76    for outer_scope in scopes:
     77        for table, inner_scope in outer_scope.selected_sources.values():
     78            if isinstance(inner_scope, Scope) and inner_scope.is_cte:
     79                cte_selections[id(inner_scope)].append(
     80                    (
     81                        outer_scope,
     82                        inner_scope,
     83                        table,
     84                    )
     85                )
     86
     87    singular_cte_selections = [v[0] for k, v in cte_selections.items() if len(v) == 1]
     88    for outer_scope, inner_scope, table in singular_cte_selections:
     89        from_or_join = table.find_ancestor(exp.From, exp.Join)
     90        if _mergeable(outer_scope, inner_scope, leave_tables_isolated, from_or_join):
     91            alias = table.alias_or_name
     92            _rename_inner_sources(outer_scope, inner_scope, alias)
     93            _merge_from(outer_scope, inner_scope, table, alias)
     94            _merge_expressions(outer_scope, inner_scope, alias)
     95            _merge_joins(outer_scope, inner_scope, from_or_join)
     96            _merge_where(outer_scope, inner_scope, from_or_join)
     97            _merge_order(outer_scope, inner_scope)
     98            _merge_hints(outer_scope, inner_scope)
     99            _pop_cte(inner_scope)
    100            outer_scope.clear_cache()
    101    return expression
    102
    103
    104def merge_derived_tables(expression: E, leave_tables_isolated: bool = False) -> E:
    105    for outer_scope in traverse_scope(expression):
    106        for subquery in outer_scope.derived_tables:
    107            from_or_join = subquery.find_ancestor(exp.From, exp.Join)
    108            alias = subquery.alias_or_name
    109            inner_scope = outer_scope.sources[alias]
    110            if _mergeable(outer_scope, inner_scope, leave_tables_isolated, from_or_join):
    111                _rename_inner_sources(outer_scope, inner_scope, alias)
    112                _merge_from(outer_scope, inner_scope, subquery, alias)
    113                _merge_expressions(outer_scope, inner_scope, alias)
    114                _merge_joins(outer_scope, inner_scope, from_or_join)
    115                _merge_where(outer_scope, inner_scope, from_or_join)
    116                _merge_order(outer_scope, inner_scope)
    117                _merge_hints(outer_scope, inner_scope)
    118                outer_scope.clear_cache()
    119
    120    return expression
    121
    122
    123def _mergeable(
    124    outer_scope: Scope, inner_scope: Scope, leave_tables_isolated: bool, from_or_join: FromOrJoin
    125) -> bool:
    126    """
    127    Return True if `inner_select` can be merged into outer query.
    128    """
    129    inner_select = inner_scope.expression.unnest()
    130
    131    def _is_a_window_expression_in_unmergable_operation():
    132        window_expressions = inner_select.find_all(exp.Window)
    133        window_alias_names = {window.parent.alias_or_name for window in window_expressions}
    134        inner_select_name = from_or_join.alias_or_name
    135        unmergable_window_columns = [
    136            column
    137            for column in outer_scope.columns
    138            if column.find_ancestor(
    139                exp.Where, exp.Group, exp.Order, exp.Join, exp.Having, exp.AggFunc
    140            )
    141        ]
    142        window_expressions_in_unmergable = [
    143            column
    144            for column in unmergable_window_columns
    145            if column.table == inner_select_name and column.name in window_alias_names
    146        ]
    147        return any(window_expressions_in_unmergable)
    148
    149    def _outer_select_joins_on_inner_select_join():
    150        """
    151        All columns from the inner select in the ON clause must be from the first FROM table.
    152
    153        That is, this can be merged:
    154            SELECT * FROM x JOIN (SELECT y.a AS a FROM y JOIN z) AS q ON x.a = q.a
    155                                         ^^^           ^
    156        But this can't:
    157            SELECT * FROM x JOIN (SELECT z.a AS a FROM y JOIN z) AS q ON x.a = q.a
    158                                         ^^^                  ^
    159        """
    160        if not isinstance(from_or_join, exp.Join):
    161            return False
    162
    163        alias = from_or_join.alias_or_name
    164
    165        on = from_or_join.args.get("on")
    166        if not on:
    167            return False
    168        selections = [c.name for c in on.find_all(exp.Column) if c.table == alias]
    169        inner_from = inner_scope.expression.args.get("from")
    170        if not inner_from:
    171            return False
    172        inner_from_table = inner_from.alias_or_name
    173        inner_projections = {s.alias_or_name: s for s in inner_scope.expression.selects}
    174        return any(
    175            col.table != inner_from_table
    176            for selection in selections
    177            for col in inner_projections[selection].find_all(exp.Column)
    178        )
    179
    180    def _is_recursive():
    181        # Recursive CTEs look like this:
    182        #     WITH RECURSIVE cte AS (
    183        #       SELECT * FROM x  <-- inner scope
    184        #       UNION ALL
    185        #       SELECT * FROM cte  <-- outer scope
    186        #     )
    187        cte = inner_scope.expression.parent
    188        node = outer_scope.expression.parent
    189
    190        while node:
    191            if node is cte:
    192                return True
    193            node = node.parent
    194        return False
    195
    196    return (
    197        isinstance(outer_scope.expression, exp.Select)
    198        and not outer_scope.expression.is_star
    199        and isinstance(inner_select, exp.Select)
    200        and not any(inner_select.args.get(arg) for arg in UNMERGABLE_ARGS)
    201        and inner_select.args.get("from") is not None
    202        and not outer_scope.pivots
    203        and not any(e.find(exp.AggFunc, exp.Select, exp.Explode) for e in inner_select.expressions)
    204        and not (leave_tables_isolated and len(outer_scope.selected_sources) > 1)
    205        and not (
    206            isinstance(from_or_join, exp.Join)
    207            and inner_select.args.get("where")
    208            and from_or_join.side in ("FULL", "LEFT", "RIGHT")
    209        )
    210        and not (
    211            isinstance(from_or_join, exp.From)
    212            and inner_select.args.get("where")
    213            and any(
    214                j.side in ("FULL", "RIGHT") for j in outer_scope.expression.args.get("joins", [])
    215            )
    216        )
    217        and not _outer_select_joins_on_inner_select_join()
    218        and not _is_a_window_expression_in_unmergable_operation()
    219        and not _is_recursive()
    220        and not (inner_select.args.get("order") and outer_scope.is_union)
    221    )
    222
    223
    224def _rename_inner_sources(outer_scope: Scope, inner_scope: Scope, alias: str) -> None:
    225    """
    226    Renames any sources in the inner query that conflict with names in the outer query.
    227    """
    228    inner_taken = set(inner_scope.selected_sources)
    229    outer_taken = set(outer_scope.selected_sources)
    230    conflicts = outer_taken.intersection(inner_taken)
    231    conflicts -= {alias}
    232
    233    taken = outer_taken.union(inner_taken)
    234
    235    for conflict in conflicts:
    236        new_name = find_new_name(taken, conflict)
    237
    238        source, _ = inner_scope.selected_sources[conflict]
    239        new_alias = exp.to_identifier(new_name)
    240
    241        if isinstance(source, exp.Subquery):
    242            source.set("alias", exp.TableAlias(this=new_alias))
    243        elif isinstance(source, exp.Table) and source.alias:
    244            source.set("alias", new_alias)
    245        elif isinstance(source, exp.Table):
    246            source.replace(exp.alias_(source, new_alias))
    247
    248        for column in inner_scope.source_columns(conflict):
    249            column.set("table", exp.to_identifier(new_name))
    250
    251        inner_scope.rename_source(conflict, new_name)
    252
    253
    254def _merge_from(
    255    outer_scope: Scope,
    256    inner_scope: Scope,
    257    node_to_replace: t.Union[exp.Subquery, exp.Table],
    258    alias: str,
    259) -> None:
    260    """
    261    Merge FROM clause of inner query into outer query.
    262    """
    263    new_subquery = inner_scope.expression.args["from"].this
    264    new_subquery.set("joins", node_to_replace.args.get("joins"))
    265    node_to_replace.replace(new_subquery)
    266    for join_hint in outer_scope.join_hints:
    267        tables = join_hint.find_all(exp.Table)
    268        for table in tables:
    269            if table.alias_or_name == node_to_replace.alias_or_name:
    270                table.set("this", exp.to_identifier(new_subquery.alias_or_name))
    271    outer_scope.remove_source(alias)
    272    outer_scope.add_source(
    273        new_subquery.alias_or_name, inner_scope.sources[new_subquery.alias_or_name]
    274    )
    275
    276
    277def _merge_joins(outer_scope: Scope, inner_scope: Scope, from_or_join: FromOrJoin) -> None:
    278    """
    279    Merge JOIN clauses of inner query into outer query.
    280    """
    281
    282    new_joins = []
    283
    284    joins = inner_scope.expression.args.get("joins") or []
    285    for join in joins:
    286        new_joins.append(join)
    287        outer_scope.add_source(join.alias_or_name, inner_scope.sources[join.alias_or_name])
    288
    289    if new_joins:
    290        outer_joins = outer_scope.expression.args.get("joins", [])
    291
    292        # Maintain the join order
    293        if isinstance(from_or_join, exp.From):
    294            position = 0
    295        else:
    296            position = outer_joins.index(from_or_join) + 1
    297        outer_joins[position:position] = new_joins
    298
    299        outer_scope.expression.set("joins", outer_joins)
    300
    301
    302def _merge_expressions(outer_scope: Scope, inner_scope: Scope, alias: str) -> None:
    303    """
    304    Merge projections of inner query into outer query.
    305
    306    Args:
    307        outer_scope (sqlglot.optimizer.scope.Scope)
    308        inner_scope (sqlglot.optimizer.scope.Scope)
    309        alias (str)
    310    """
    311    # Collect all columns that reference the alias of the inner query
    312    outer_columns = defaultdict(list)
    313    for column in outer_scope.columns:
    314        if column.table == alias:
    315            outer_columns[column.name].append(column)
    316
    317    # Replace columns with the projection expression in the inner query
    318    for expression in inner_scope.expression.expressions:
    319        projection_name = expression.alias_or_name
    320        if not projection_name:
    321            continue
    322        columns_to_replace = outer_columns.get(projection_name, [])
    323
    324        expression = expression.unalias()
    325        must_wrap_expression = not isinstance(expression, SAFE_TO_REPLACE_UNWRAPPED)
    326
    327        for column in columns_to_replace:
    328            # Ensures we don't alter the intended operator precedence if there's additional
    329            # context surrounding the outer expression (i.e. it's not a simple projection).
    330            if isinstance(column.parent, (exp.Unary, exp.Binary)) and must_wrap_expression:
    331                expression = exp.paren(expression, copy=False)
    332
    333            column.replace(expression.copy())
    334
    335
    336def _merge_where(outer_scope: Scope, inner_scope: Scope, from_or_join: FromOrJoin) -> None:
    337    """
    338    Merge WHERE clause of inner query into outer query.
    339
    340    Args:
    341        outer_scope (sqlglot.optimizer.scope.Scope)
    342        inner_scope (sqlglot.optimizer.scope.Scope)
    343        from_or_join (exp.From|exp.Join)
    344    """
    345    where = inner_scope.expression.args.get("where")
    346    if not where or not where.this:
    347        return
    348
    349    expression = outer_scope.expression
    350
    351    if isinstance(from_or_join, exp.Join):
    352        # Merge predicates from an outer join to the ON clause
    353        # if it only has columns that are already joined
    354        from_ = expression.args.get("from")
    355        sources = {from_.alias_or_name} if from_ else set()
    356
    357        for join in expression.args["joins"]:
    358            source = join.alias_or_name
    359            sources.add(source)
    360            if source == from_or_join.alias_or_name:
    361                break
    362
    363        if exp.column_table_names(where.this) <= sources:
    364            from_or_join.on(where.this, copy=False)
    365            from_or_join.set("on", from_or_join.args.get("on"))
    366            return
    367
    368    expression.where(where.this, copy=False)
    369
    370
    371def _merge_order(outer_scope: Scope, inner_scope: Scope) -> None:
    372    """
    373    Merge ORDER clause of inner query into outer query.
    374
    375    Args:
    376        outer_scope (sqlglot.optimizer.scope.Scope)
    377        inner_scope (sqlglot.optimizer.scope.Scope)
    378    """
    379    if (
    380        any(
    381            outer_scope.expression.args.get(arg) for arg in ["group", "distinct", "having", "order"]
    382        )
    383        or len(outer_scope.selected_sources) != 1
    384        or any(expression.find(exp.AggFunc) for expression in outer_scope.expression.expressions)
    385    ):
    386        return
    387
    388    outer_scope.expression.set("order", inner_scope.expression.args.get("order"))
    389
    390
    391def _merge_hints(outer_scope: Scope, inner_scope: Scope) -> None:
    392    inner_scope_hint = inner_scope.expression.args.get("hint")
    393    if not inner_scope_hint:
    394        return
    395    outer_scope_hint = outer_scope.expression.args.get("hint")
    396    if outer_scope_hint:
    397        for hint_expression in inner_scope_hint.expressions:
    398            outer_scope_hint.append("expressions", hint_expression)
    399    else:
    400        outer_scope.expression.set("hint", inner_scope_hint)
    401
    402
    403def _pop_cte(inner_scope: Scope) -> None:
    404    """
    405    Remove CTE from the AST.
    406
    407    Args:
    408        inner_scope (sqlglot.optimizer.scope.Scope)
    409    """
    410    cte = inner_scope.expression.parent
    411    with_ = cte.parent
    412    if len(with_.expressions) == 1:
    413        with_.pop()
    414    else:
    415        cte.pop()

def merge_subqueries(expression: ~E, leave_tables_isolated: bool = False) -> ~E: View Source

    18def merge_subqueries(expression: E, leave_tables_isolated: bool = False) -> E:
    19    """
    20    Rewrite sqlglot AST to merge derived tables into the outer query.
    21
    22    This also merges CTEs if they are selected from only once.
    23
    24    Example:
    25        >>> import sqlglot
    26        >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT x.a FROM x) CROSS JOIN y")
    27        >>> merge_subqueries(expression).sql()
    28        'SELECT x.a FROM x CROSS JOIN y'
    29
    30    If `leave_tables_isolated` is True, this will not merge inner queries into outer
    31    queries if it would result in multiple table selects in a single query:
    32        >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT x.a FROM x) CROSS JOIN y")
    33        >>> merge_subqueries(expression, leave_tables_isolated=True).sql()
    34        'SELECT a FROM (SELECT x.a FROM x) CROSS JOIN y'
    35
    36    Inspired by https://dev.mysql.com/doc/refman/8.0/en/derived-table-optimization.html
    37
    38    Args:
    39        expression (sqlglot.Expression): expression to optimize
    40        leave_tables_isolated (bool):
    41    Returns:
    42        sqlglot.Expression: optimized expression
    43    """
    44    expression = merge_ctes(expression, leave_tables_isolated)
    45    expression = merge_derived_tables(expression, leave_tables_isolated)
    46    return expression

Rewrite sqlglot AST to merge derived tables into the outer query.

This also merges CTEs if they are selected from only once.

###### Example:

>     >>> import sqlglot
>     >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT x.a FROM x) CROSS JOIN y")
>     >>> merge_subqueries(expression).sql()
>     'SELECT x.a FROM x CROSS JOIN y'

If `leave_tables_isolated` is True, this will not merge inner queries into outer queries if it would result in multiple table selects in a single query:

> > > expression = sqlglot.parse_one("SELECT a FROM (SELECT x.a FROM x) CROSS JOIN y") merge_subqueries(expression, leave_tables_isolated=True).sql() 'SELECT a FROM (SELECT x.a FROM x) CROSS JOIN y'

Inspired by https://dev.mysql.com/doc/refman/8.0/en/derived-table-optimization.html

###### Arguments:

- expression (sqlglot.Expression): expression to optimize
- leave_tables_isolated (bool):

###### Returns:

> sqlglot.Expression: optimized expression

UNMERGABLE_ARGS = {'qualify', 'prewhere', 'windows', 'options', 'operation_modifiers', 'settings', 'match', 'with', 'pivots', 'locks', 'having', 'sample', 'format', 'limit', 'distribute', 'sort', 'connect', 'kind', 'group', 'offset', 'distinct', 'into', 'laterals', 'cluster'}
SAFE_TO_REPLACE_UNWRAPPED = (<class 'sqlglot.expressions.Column'>, <class 'sqlglot.expressions.EQ'>, <class 'sqlglot.expressions.Func'>, <class 'sqlglot.expressions.NEQ'>, <class 'sqlglot.expressions.Paren'>)
def merge_ctes(expression: ~E, leave_tables_isolated: bool = False) -> ~E: View Source

     71def merge_ctes(expression: E, leave_tables_isolated: bool = False) -> E:
     72    scopes = traverse_scope(expression)
     73
     74    # All places where we select from CTEs.
     75    # We key on the CTE scope so we can detect CTES that are selected from multiple times.
     76    cte_selections = defaultdict(list)
     77    for outer_scope in scopes:
     78        for table, inner_scope in outer_scope.selected_sources.values():
     79            if isinstance(inner_scope, Scope) and inner_scope.is_cte:
     80                cte_selections[id(inner_scope)].append(
     81                    (
     82                        outer_scope,
     83                        inner_scope,
     84                        table,
     85                    )
     86                )
     87
     88    singular_cte_selections = [v[0] for k, v in cte_selections.items() if len(v) == 1]
     89    for outer_scope, inner_scope, table in singular_cte_selections:
     90        from_or_join = table.find_ancestor(exp.From, exp.Join)
     91        if _mergeable(outer_scope, inner_scope, leave_tables_isolated, from_or_join):
     92            alias = table.alias_or_name
     93            _rename_inner_sources(outer_scope, inner_scope, alias)
     94            _merge_from(outer_scope, inner_scope, table, alias)
     95            _merge_expressions(outer_scope, inner_scope, alias)
     96            _merge_joins(outer_scope, inner_scope, from_or_join)
     97            _merge_where(outer_scope, inner_scope, from_or_join)
     98            _merge_order(outer_scope, inner_scope)
     99            _merge_hints(outer_scope, inner_scope)
    100            _pop_cte(inner_scope)
    101            outer_scope.clear_cache()
    102    return expression

def merge_derived_tables(expression: ~E, leave_tables_isolated: bool = False) -> ~E: View Source

    105def merge_derived_tables(expression: E, leave_tables_isolated: bool = False) -> E:
    106    for outer_scope in traverse_scope(expression):
    107        for subquery in outer_scope.derived_tables:
    108            from_or_join = subquery.find_ancestor(exp.From, exp.Join)
    109            alias = subquery.alias_or_name
    110            inner_scope = outer_scope.sources[alias]
    111            if _mergeable(outer_scope, inner_scope, leave_tables_isolated, from_or_join):
    112                _rename_inner_sources(outer_scope, inner_scope, alias)
    113                _merge_from(outer_scope, inner_scope, subquery, alias)
    114                _merge_expressions(outer_scope, inner_scope, alias)
    115                _merge_joins(outer_scope, inner_scope, from_or_join)
    116                _merge_where(outer_scope, inner_scope, from_or_join)
    117                _merge_order(outer_scope, inner_scope)
    118                _merge_hints(outer_scope, inner_scope)
    119                outer_scope.clear_cache()
    120
    121    return expression