sqlglot.optimizer.pushdown_projections

  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()