Edit on GitHub

# sqlglot.optimizer.merge_subqueries

View Source

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

def merge_subqueries(expression, leave_tables_isolated=False): View Source

     9def merge_subqueries(expression, leave_tables_isolated=False):
    10    """
    11    Rewrite sqlglot AST to merge derived tables into the outer query.
    12
    13    This also merges CTEs if they are selected from only once.
    14
    15    Example:
    16        >>> import sqlglot
    17        >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT x.a FROM x) CROSS JOIN y")
    18        >>> merge_subqueries(expression).sql()
    19        'SELECT x.a FROM x CROSS JOIN y'
    20
    21    If `leave_tables_isolated` is True, this will not merge inner queries into outer
    22    queries if it would result in multiple table selects in a single query:
    23        >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT x.a FROM x) CROSS JOIN y")
    24        >>> merge_subqueries(expression, leave_tables_isolated=True).sql()
    25        'SELECT a FROM (SELECT x.a FROM x) CROSS JOIN y'
    26
    27    Inspired by https://dev.mysql.com/doc/refman/8.0/en/derived-table-optimization.html
    28
    29    Args:
    30        expression (sqlglot.Expression): expression to optimize
    31        leave_tables_isolated (bool):
    32    Returns:
    33        sqlglot.Expression: optimized expression
    34    """
    35    expression = merge_ctes(expression, leave_tables_isolated)
    36    expression = merge_derived_tables(expression, leave_tables_isolated)
    37    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 = {'offset', 'prewhere', 'match', 'locks', 'qualify', 'windows', 'pivots', 'cluster', 'settings', 'having', 'group', 'options', 'distinct', 'with', 'distribute', 'sample', 'format', 'connect', 'laterals', 'limit', 'sort', 'into', 'kind'}
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, leave_tables_isolated=False): View Source

    62def merge_ctes(expression, leave_tables_isolated=False):
    63    scopes = traverse_scope(expression)
    64
    65    # All places where we select from CTEs.
    66    # We key on the CTE scope so we can detect CTES that are selected from multiple times.
    67    cte_selections = defaultdict(list)
    68    for outer_scope in scopes:
    69        for table, inner_scope in outer_scope.selected_sources.values():
    70            if isinstance(inner_scope, Scope) and inner_scope.is_cte:
    71                cte_selections[id(inner_scope)].append(
    72                    (
    73                        outer_scope,
    74                        inner_scope,
    75                        table,
    76                    )
    77                )
    78
    79    singular_cte_selections = [v[0] for k, v in cte_selections.items() if len(v) == 1]
    80    for outer_scope, inner_scope, table in singular_cte_selections:
    81        from_or_join = table.find_ancestor(exp.From, exp.Join)
    82        if _mergeable(outer_scope, inner_scope, leave_tables_isolated, from_or_join):
    83            alias = table.alias_or_name
    84            _rename_inner_sources(outer_scope, inner_scope, alias)
    85            _merge_from(outer_scope, inner_scope, table, alias)
    86            _merge_expressions(outer_scope, inner_scope, alias)
    87            _merge_joins(outer_scope, inner_scope, from_or_join)
    88            _merge_where(outer_scope, inner_scope, from_or_join)
    89            _merge_order(outer_scope, inner_scope)
    90            _merge_hints(outer_scope, inner_scope)
    91            _pop_cte(inner_scope)
    92            outer_scope.clear_cache()
    93    return expression

def merge_derived_tables(expression, leave_tables_isolated=False): View Source

     96def merge_derived_tables(expression, leave_tables_isolated=False):
     97    for outer_scope in traverse_scope(expression):
     98        for subquery in outer_scope.derived_tables:
     99            from_or_join = subquery.find_ancestor(exp.From, exp.Join)
    100            alias = subquery.alias_or_name
    101            inner_scope = outer_scope.sources[alias]
    102            if _mergeable(outer_scope, inner_scope, leave_tables_isolated, from_or_join):
    103                _rename_inner_sources(outer_scope, inner_scope, alias)
    104                _merge_from(outer_scope, inner_scope, subquery, alias)
    105                _merge_expressions(outer_scope, inner_scope, alias)
    106                _merge_joins(outer_scope, inner_scope, from_or_join)
    107                _merge_where(outer_scope, inner_scope, from_or_join)
    108                _merge_order(outer_scope, inner_scope)
    109                _merge_hints(outer_scope, inner_scope)
    110                outer_scope.clear_cache()
    111
    112    return expression