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# sqlglot.optimizer.pushdown_projections

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      1from collections import defaultdict
      2
      3from sqlglot import alias, exp
      4from sqlglot.optimizer.qualify_columns import Resolver
      5from sqlglot.optimizer.scope import Scope, traverse_scope
      6from sqlglot.schema import ensure_schema
      7
      8# Sentinel value that means an outer query selecting ALL columns
      9SELECT_ALL = object()
     10
     11# Selection to use if selection list is empty
     12DEFAULT_SELECTION = lambda is_agg: alias(
     13    exp.Max(this=exp.Literal.number(1)) if is_agg else "1", "_"
     14)
     15
     16
     17def pushdown_projections(expression, schema=None, remove_unused_selections=True):
     18    """
     19    Rewrite sqlglot AST to remove unused columns projections.
     20
     21    Example:
     22        >>> import sqlglot
     23        >>> sql = "SELECT y.a AS a FROM (SELECT x.a AS a, x.b AS b FROM x) AS y"
     24        >>> expression = sqlglot.parse_one(sql)
     25        >>> pushdown_projections(expression).sql()
     26        'SELECT y.a AS a FROM (SELECT x.a AS a FROM x) AS y'
     27
     28    Args:
     29        expression (sqlglot.Expression): expression to optimize
     30        remove_unused_selections (bool): remove selects that are unused
     31    Returns:
     32        sqlglot.Expression: optimized expression
     33    """
     34    # Map of Scope to all columns being selected by outer queries.
     35    schema = ensure_schema(schema)
     36    source_column_alias_count = {}
     37    referenced_columns = defaultdict(set)
     38
     39    # We build the scope tree (which is traversed in DFS postorder), then iterate
     40    # over the result in reverse order. This should ensure that the set of selected
     41    # columns for a particular scope are completely build by the time we get to it.
     42    for scope in reversed(traverse_scope(expression)):
     43        parent_selections = referenced_columns.get(scope, {SELECT_ALL})
     44        alias_count = source_column_alias_count.get(scope, 0)
     45
     46        if scope.expression.args.get("distinct") or (scope.parent and scope.parent.pivots):
     47            # We can't remove columns SELECT DISTINCT nor UNION DISTINCT. The same holds if
     48            # we select from a pivoted source in the parent scope.
     49            parent_selections = {SELECT_ALL}
     50
     51        if isinstance(scope.expression, exp.Union):
     52            left, right = scope.union_scopes
     53            referenced_columns[left] = parent_selections
     54
     55            if any(select.is_star for select in right.expression.selects):
     56                referenced_columns[right] = parent_selections
     57            elif not any(select.is_star for select in left.expression.selects):
     58                referenced_columns[right] = [
     59                    right.expression.selects[i].alias_or_name
     60                    for i, select in enumerate(left.expression.selects)
     61                    if SELECT_ALL in parent_selections or select.alias_or_name in parent_selections
     62                ]
     63
     64        if isinstance(scope.expression, exp.Select):
     65            if remove_unused_selections:
     66                _remove_unused_selections(scope, parent_selections, schema, alias_count)
     67
     68            if scope.expression.is_star:
     69                continue
     70
     71            # Group columns by source name
     72            selects = defaultdict(set)
     73            for col in scope.columns:
     74                table_name = col.table
     75                col_name = col.name
     76                selects[table_name].add(col_name)
     77
     78            # Push the selected columns down to the next scope
     79            for name, (node, source) in scope.selected_sources.items():
     80                if isinstance(source, Scope):
     81                    columns = selects.get(name) or set()
     82                    referenced_columns[source].update(columns)
     83
     84                column_aliases = node.alias_column_names
     85                if column_aliases:
     86                    source_column_alias_count[source] = len(column_aliases)
     87
     88    return expression
     89
     90
     91def _remove_unused_selections(scope, parent_selections, schema, alias_count):
     92    order = scope.expression.args.get("order")
     93
     94    if order:
     95        # Assume columns without a qualified table are references to output columns
     96        order_refs = {c.name for c in order.find_all(exp.Column) if not c.table}
     97    else:
     98        order_refs = set()
     99
    100    new_selections = []
    101    removed = False
    102    star = False
    103    is_agg = False
    104
    105    select_all = SELECT_ALL in parent_selections
    106
    107    for selection in scope.expression.selects:
    108        name = selection.alias_or_name
    109
    110        if select_all or name in parent_selections or name in order_refs or alias_count > 0:
    111            new_selections.append(selection)
    112            alias_count -= 1
    113        else:
    114            if selection.is_star:
    115                star = True
    116            removed = True
    117
    118        if not is_agg and selection.find(exp.AggFunc):
    119            is_agg = True
    120
    121    if star:
    122        resolver = Resolver(scope, schema)
    123        names = {s.alias_or_name for s in new_selections}
    124
    125        for name in sorted(parent_selections):
    126            if name not in names:
    127                new_selections.append(
    128                    alias(exp.column(name, table=resolver.get_table(name)), name, copy=False)
    129                )
    130
    131    # If there are no remaining selections, just select a single constant
    132    if not new_selections:
    133        new_selections.append(DEFAULT_SELECTION(is_agg))
    134
    135    scope.expression.select(*new_selections, append=False, copy=False)
    136
    137    if removed:
    138        scope.clear_cache()

SELECT_ALL = <object object>
def DEFAULT_SELECTION(is_agg): View Source

    13DEFAULT_SELECTION = lambda is_agg: alias(
    14    exp.Max(this=exp.Literal.number(1)) if is_agg else "1", "_"
    15)

def pushdown_projections(expression, schema=None, remove_unused_selections=True): View Source

    18def pushdown_projections(expression, schema=None, remove_unused_selections=True):
    19    """
    20    Rewrite sqlglot AST to remove unused columns projections.
    21
    22    Example:
    23        >>> import sqlglot
    24        >>> sql = "SELECT y.a AS a FROM (SELECT x.a AS a, x.b AS b FROM x) AS y"
    25        >>> expression = sqlglot.parse_one(sql)
    26        >>> pushdown_projections(expression).sql()
    27        'SELECT y.a AS a FROM (SELECT x.a AS a FROM x) AS y'
    28
    29    Args:
    30        expression (sqlglot.Expression): expression to optimize
    31        remove_unused_selections (bool): remove selects that are unused
    32    Returns:
    33        sqlglot.Expression: optimized expression
    34    """
    35    # Map of Scope to all columns being selected by outer queries.
    36    schema = ensure_schema(schema)
    37    source_column_alias_count = {}
    38    referenced_columns = defaultdict(set)
    39
    40    # We build the scope tree (which is traversed in DFS postorder), then iterate
    41    # over the result in reverse order. This should ensure that the set of selected
    42    # columns for a particular scope are completely build by the time we get to it.
    43    for scope in reversed(traverse_scope(expression)):
    44        parent_selections = referenced_columns.get(scope, {SELECT_ALL})
    45        alias_count = source_column_alias_count.get(scope, 0)
    46
    47        if scope.expression.args.get("distinct") or (scope.parent and scope.parent.pivots):
    48            # We can't remove columns SELECT DISTINCT nor UNION DISTINCT. The same holds if
    49            # we select from a pivoted source in the parent scope.
    50            parent_selections = {SELECT_ALL}
    51
    52        if isinstance(scope.expression, exp.Union):
    53            left, right = scope.union_scopes
    54            referenced_columns[left] = parent_selections
    55
    56            if any(select.is_star for select in right.expression.selects):
    57                referenced_columns[right] = parent_selections
    58            elif not any(select.is_star for select in left.expression.selects):
    59                referenced_columns[right] = [
    60                    right.expression.selects[i].alias_or_name
    61                    for i, select in enumerate(left.expression.selects)
    62                    if SELECT_ALL in parent_selections or select.alias_or_name in parent_selections
    63                ]
    64
    65        if isinstance(scope.expression, exp.Select):
    66            if remove_unused_selections:
    67                _remove_unused_selections(scope, parent_selections, schema, alias_count)
    68
    69            if scope.expression.is_star:
    70                continue
    71
    72            # Group columns by source name
    73            selects = defaultdict(set)
    74            for col in scope.columns:
    75                table_name = col.table
    76                col_name = col.name
    77                selects[table_name].add(col_name)
    78
    79            # Push the selected columns down to the next scope
    80            for name, (node, source) in scope.selected_sources.items():
    81                if isinstance(source, Scope):
    82                    columns = selects.get(name) or set()
    83                    referenced_columns[source].update(columns)
    84
    85                column_aliases = node.alias_column_names
    86                if column_aliases:
    87                    source_column_alias_count[source] = len(column_aliases)
    88
    89    return expression

Rewrite sqlglot AST to remove unused columns projections.

###### Example:

>     >>> import sqlglot
>     >>> sql = "SELECT y.a AS a FROM (SELECT x.a AS a, x.b AS b FROM x) AS y"
>     >>> expression = sqlglot.parse_one(sql)
>     >>> pushdown_projections(expression).sql()
>     'SELECT y.a AS a FROM (SELECT x.a AS a FROM x) AS y'

###### Arguments:

- expression (sqlglot.Expression): expression to optimize
- remove_unused_selections (bool): remove selects that are unused

###### Returns:

> sqlglot.Expression: optimized expression