# sqlglot.optimizer.pushdown_projections API documentation

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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
     12# Selection to use if selection list is empty
     13def default_selection(is_agg: bool) -> exp.Alias:
     14    return alias(exp.Max(this=exp.Literal.number(1)) if is_agg else "1", "_")
     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        # We can't remove columns SELECT DISTINCT nor UNION DISTINCT.
     47        if scope.expression.args.get("distinct"):
     48            parent_selections = {SELECT_ALL}
     49
     50        if isinstance(scope.expression, exp.Union):
     51            left, right = scope.union_scopes
     52            referenced_columns[left] = parent_selections
     53
     54            if any(select.is_star for select in right.expression.selects):
     55                referenced_columns[right] = parent_selections
     56            elif not any(select.is_star for select in left.expression.selects):
     57                referenced_columns[right] = [
     58                    right.expression.selects[i].alias_or_name
     59                    for i, select in enumerate(left.expression.selects)
     60                    if SELECT_ALL in parent_selections or select.alias_or_name in parent_selections
     61                ]
     62
     63        if isinstance(scope.expression, exp.Select):
     64            if remove_unused_selections:
     65                _remove_unused_selections(scope, parent_selections, schema, alias_count)
     66
     67            if scope.expression.is_star:
     68                continue
     69
     70            # Group columns by source name
     71            selects = defaultdict(set)
     72            for col in scope.columns:
     73                table_name = col.table
     74                col_name = col.name
     75                selects[table_name].add(col_name)
     76
     77            # Push the selected columns down to the next scope
     78            for name, (node, source) in scope.selected_sources.items():
     79                if isinstance(source, Scope):
     80                    columns = {SELECT_ALL} if scope.pivots else selects.get(name) or set()
     81                    referenced_columns[source].update(columns)
     82
     83                column_aliases = node.alias_column_names
     84                if column_aliases:
     85                    source_column_alias_count[source] = len(column_aliases)
     86
     87    return expression
     88
     89
     90def _remove_unused_selections(scope, parent_selections, schema, alias_count):
     91    order = scope.expression.args.get("order")
     92
     93    if order:
     94        # Assume columns without a qualified table are references to output columns
     95        order_refs = {c.name for c in order.find_all(exp.Column) if not c.table}
     96    else:
     97        order_refs = set()
     98
     99    new_selections = []
    100    removed = False
    101    star = False
    102    is_agg = False
    103
    104    select_all = SELECT_ALL in parent_selections
    105
    106    for selection in scope.expression.selects:
    107        name = selection.alias_or_name
    108
    109        if select_all or name in parent_selections or name in order_refs or alias_count > 0:
    110            new_selections.append(selection)
    111            alias_count -= 1
    112        else:
    113            if selection.is_star:
    114                star = True
    115            removed = True
    116
    117        if not is_agg and selection.find(exp.AggFunc):
    118            is_agg = True
    119
    120    if star:
    121        resolver = Resolver(scope, schema)
    122        names = {s.alias_or_name for s in new_selections}
    123
    124        for name in sorted(parent_selections):
    125            if name not in names:
    126                new_selections.append(
    127                    alias(exp.column(name, table=resolver.get_table(name)), name, copy=False)
    128                )
    129
    130    # If there are no remaining selections, just select a single constant
    131    if not new_selections:
    132        new_selections.append(default_selection(is_agg))
    133
    134    scope.expression.select(*new_selections, append=False, copy=False)
    135
    136    if removed:
    137        scope.clear_cache()

SELECT_ALL = <object object>
def default_selection(is_agg: bool) -> sqlglot.expressions.Alias: View Source

    14def default_selection(is_agg: bool) -> exp.Alias:
    15    return alias(exp.Max(this=exp.Literal.number(1)) if is_agg else "1", "_")

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        # We can't remove columns SELECT DISTINCT nor UNION DISTINCT.
    48        if scope.expression.args.get("distinct"):
    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 = {SELECT_ALL} if scope.pivots else 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

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