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from collections import defaultdict
from sqlglot import alias, exp
from sqlglot.optimizer.scope import Scope, traverse_scope
# Sentinel value that means an outer query selecting ALL columns
SELECT_ALL = object()
def pushdown_projections(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'
Args:
expression (sqlglot.Expression): expression to optimize
Returns:
sqlglot.Expression: optimized expression
"""
# Map of Scope to all columns being selected by outer queries.
referenced_columns = defaultdict(set)
# We build the scope tree (which is traversed in DFS postorder), then iterate
# over the result in reverse order. This should ensure that the set of selected
# columns for a particular scope are completely build by the time we get to it.
for scope in reversed(traverse_scope(expression)):
parent_selections = referenced_columns.get(scope, {SELECT_ALL})
if scope.expression.args.get("distinct"):
# We can't remove columns SELECT DISTINCT nor UNION DISTINCT
parent_selections = {SELECT_ALL}
if isinstance(scope.expression, exp.Union):
left, right = scope.union_scopes
referenced_columns[left] = parent_selections
referenced_columns[right] = parent_selections
if isinstance(scope.expression, exp.Select):
_remove_unused_selections(scope, parent_selections)
# Group columns by source name
selects = defaultdict(set)
for col in scope.columns:
table_name = col.table
col_name = col.name
selects[table_name].add(col_name)
# Push the selected columns down to the next scope
for name, (_, source) in scope.selected_sources.items():
if isinstance(source, Scope):
columns = selects.get(name) or set()
referenced_columns[source].update(columns)
return expression
def _remove_unused_selections(scope, parent_selections):
order = scope.expression.args.get("order")
if order:
# Assume columns without a qualified table are references to output columns
order_refs = {c.name for c in order.find_all(exp.Column) if not c.table}
else:
order_refs = set()
new_selections = []
for selection in scope.selects:
if (
SELECT_ALL in parent_selections
or selection.alias_or_name in parent_selections
or selection.alias_or_name in order_refs
):
new_selections.append(selection)
# If there are no remaining selections, just select a single constant
if not new_selections:
new_selections.append(alias("1", "_"))
scope.expression.set("expressions", new_selections)
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