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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()
# SELECTION TO USE IF SELECTION LIST IS EMPTY
DEFAULT_SELECTION = alias("1", "_")
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)
left_union = None
right_union = None
# 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_union, right_union = scope.union_scopes
referenced_columns[left_union] = parent_selections
referenced_columns[right_union] = parent_selections
if isinstance(scope.expression, exp.Select) and scope != right_union:
removed_indexes = _remove_unused_selections(scope, parent_selections)
# The left union is used for column names to select and if we remove columns from the left
# we need to also remove those same columns in the right that were at the same position
if scope is left_union:
_remove_indexed_selections(right_union, removed_indexes)
# 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):
removed_indexes = []
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 i, selection in enumerate(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)
else:
removed_indexes.append(i)
# If there are no remaining selections, just select a single constant
if not new_selections:
new_selections.append(DEFAULT_SELECTION)
scope.expression.set("expressions", new_selections)
return removed_indexes
def _remove_indexed_selections(scope, indexes_to_remove):
new_selections = [
selection for i, selection in enumerate(scope.selects) if i not in indexes_to_remove
]
if not new_selections:
new_selections.append(DEFAULT_SELECTION)
scope.expression.set("expressions", new_selections)
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