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

def DEFAULT_SELECTION(): View Source

    13DEFAULT_SELECTION = lambda: alias("1", "_")

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

    16def pushdown_projections(expression, schema=None, remove_unused_selections=True):
    17    """
    18    Rewrite sqlglot AST to remove unused columns projections.
    19
    20    Example:
    21        >>> import sqlglot
    22        >>> sql = "SELECT y.a AS a FROM (SELECT x.a AS a, x.b AS b FROM x) AS y"
    23        >>> expression = sqlglot.parse_one(sql)
    24        >>> pushdown_projections(expression).sql()
    25        'SELECT y.a AS a FROM (SELECT x.a AS a FROM x) AS y'
    26
    27    Args:
    28        expression (sqlglot.Expression): expression to optimize
    29        remove_unused_selections (bool): remove selects that are unused
    30    Returns:
    31        sqlglot.Expression: optimized expression
    32    """
    33    # Map of Scope to all columns being selected by outer queries.
    34    schema = ensure_schema(schema)
    35    referenced_columns = defaultdict(set)
    36
    37    # We build the scope tree (which is traversed in DFS postorder), then iterate
    38    # over the result in reverse order. This should ensure that the set of selected
    39    # columns for a particular scope are completely build by the time we get to it.
    40    for scope in reversed(traverse_scope(expression)):
    41        parent_selections = referenced_columns.get(scope, {SELECT_ALL})
    42
    43        if scope.expression.args.get("distinct") or scope.parent and scope.parent.pivots:
    44            # We can't remove columns SELECT DISTINCT nor UNION DISTINCT. The same holds if
    45            # we select from a pivoted source in the parent scope.
    46            parent_selections = {SELECT_ALL}
    47
    48        if isinstance(scope.expression, exp.Union):
    49            left, right = scope.union_scopes
    50            referenced_columns[left] = parent_selections
    51
    52            if any(select.is_star for select in right.selects):
    53                referenced_columns[right] = parent_selections
    54            elif not any(select.is_star for select in left.selects):
    55                referenced_columns[right] = [
    56                    right.selects[i].alias_or_name
    57                    for i, select in enumerate(left.selects)
    58                    if SELECT_ALL in parent_selections or select.alias_or_name in parent_selections
    59                ]
    60
    61        if isinstance(scope.expression, exp.Select):
    62            if remove_unused_selections:
    63                _remove_unused_selections(scope, parent_selections, schema)
    64
    65            # Group columns by source name
    66            selects = defaultdict(set)
    67            for col in scope.columns:
    68                table_name = col.table
    69                col_name = col.name
    70                selects[table_name].add(col_name)
    71
    72            # Push the selected columns down to the next scope
    73            for name, (_, source) in scope.selected_sources.items():
    74                if isinstance(source, Scope):
    75                    columns = selects.get(name) or set()
    76                    referenced_columns[source].update(columns)
    77
    78    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