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# sqlglot.optimizer.pushdown_projections

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      1from collections import defaultdict
      2
      3from sqlglot import alias, exp
      4from sqlglot.helper import flatten
      5from sqlglot.optimizer.qualify_columns import Resolver
      6from sqlglot.optimizer.scope import Scope, traverse_scope
      7from sqlglot.schema import ensure_schema
      8
      9# Sentinel value that means an outer query selecting ALL columns
     10SELECT_ALL = object()
     11
     12# Selection to use if selection list is empty
     13DEFAULT_SELECTION = lambda: alias("1", "_")
     14
     15
     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"):
     44            # We can't remove columns SELECT DISTINCT nor UNION DISTINCT
     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 = defaultdict(list)
     90    removed = False
     91    star = False
     92    for selection in scope.selects:
     93        name = selection.alias_or_name
     94
     95        if SELECT_ALL in parent_selections or name in parent_selections or name in order_refs:
     96            new_selections[name].append(selection)
     97        else:
     98            if selection.is_star:
     99                star = True
    100            removed = True
    101
    102    if star:
    103        resolver = Resolver(scope, schema)
    104
    105        for name in sorted(parent_selections):
    106            if name not in new_selections:
    107                new_selections[name].append(
    108                    alias(exp.column(name, table=resolver.get_table(name)), name)
    109                )
    110
    111    # If there are no remaining selections, just select a single constant
    112    if not new_selections:
    113        new_selections[""].append(DEFAULT_SELECTION())
    114
    115    scope.expression.select(*flatten(new_selections.values()), append=False, copy=False)
    116
    117    if removed:
    118        scope.clear_cache()

def DEFAULT_SELECTION(): View Source

    14DEFAULT_SELECTION = lambda: alias("1", "_")

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

    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    referenced_columns = defaultdict(set)
    37
    38    # We build the scope tree (which is traversed in DFS postorder), then iterate
    39    # over the result in reverse order. This should ensure that the set of selected
    40    # columns for a particular scope are completely build by the time we get to it.
    41    for scope in reversed(traverse_scope(expression)):
    42        parent_selections = referenced_columns.get(scope, {SELECT_ALL})
    43
    44        if scope.expression.args.get("distinct"):
    45            # We can't remove columns SELECT DISTINCT nor UNION DISTINCT
    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