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: alias("1", "_") 13 14 15def pushdown_projections(expression, schema=None, remove_unused_selections=True): 16 """ 17 Rewrite sqlglot AST to remove unused columns projections. 18 19 Example: 20 >>> import sqlglot 21 >>> sql = "SELECT y.a AS a FROM (SELECT x.a AS a, x.b AS b FROM x) AS y" 22 >>> expression = sqlglot.parse_one(sql) 23 >>> pushdown_projections(expression).sql() 24 'SELECT y.a AS a FROM (SELECT x.a AS a FROM x) AS y' 25 26 Args: 27 expression (sqlglot.Expression): expression to optimize 28 remove_unused_selections (bool): remove selects that are unused 29 Returns: 30 sqlglot.Expression: optimized expression 31 """ 32 # Map of Scope to all columns being selected by outer queries. 33 schema = ensure_schema(schema) 34 referenced_columns = defaultdict(set) 35 36 # We build the scope tree (which is traversed in DFS postorder), then iterate 37 # over the result in reverse order. This should ensure that the set of selected 38 # columns for a particular scope are completely build by the time we get to it. 39 for scope in reversed(traverse_scope(expression)): 40 parent_selections = referenced_columns.get(scope, {SELECT_ALL}) 41 42 if scope.expression.args.get("distinct"): 43 # We can't remove columns SELECT DISTINCT nor UNION DISTINCT 44 parent_selections = {SELECT_ALL} 45 46 if isinstance(scope.expression, exp.Union): 47 left, right = scope.union_scopes 48 referenced_columns[left] = parent_selections 49 50 if any(select.is_star for select in right.selects): 51 referenced_columns[right] = parent_selections 52 elif not any(select.is_star for select in left.selects): 53 referenced_columns[right] = [ 54 right.selects[i].alias_or_name 55 for i, select in enumerate(left.selects) 56 if SELECT_ALL in parent_selections or select.alias_or_name in parent_selections 57 ] 58 59 if isinstance(scope.expression, exp.Select): 60 if remove_unused_selections: 61 _remove_unused_selections(scope, parent_selections, schema) 62 63 # Group columns by source name 64 selects = defaultdict(set) 65 for col in scope.columns: 66 table_name = col.table 67 col_name = col.name 68 selects[table_name].add(col_name) 69 70 # Push the selected columns down to the next scope 71 for name, (_, source) in scope.selected_sources.items(): 72 if isinstance(source, Scope): 73 columns = selects.get(name) or set() 74 referenced_columns[source].update(columns) 75 76 return expression 77 78 79def _remove_unused_selections(scope, parent_selections, schema): 80 order = scope.expression.args.get("order") 81 82 if order: 83 # Assume columns without a qualified table are references to output columns 84 order_refs = {c.name for c in order.find_all(exp.Column) if not c.table} 85 else: 86 order_refs = set() 87 88 new_selections = [] 89 removed = False 90 star = False 91 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.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 names = {s.alias_or_name for s in new_selections} 105 106 for name in sorted(parent_selections): 107 if name not in names: 108 new_selections.append(alias(exp.column(name, table=resolver.get_table(name)), name)) 109 110 # If there are no remaining selections, just select a single constant 111 if not new_selections: 112 new_selections.append(DEFAULT_SELECTION()) 113 114 scope.expression.select(*new_selections, append=False, copy=False) 115 116 if removed: 117 scope.clear_cache()
def
DEFAULT_SELECTION():
13DEFAULT_SELECTION = lambda: alias("1", "_")
def
pushdown_projections(expression, schema=None, remove_unused_selections=True):
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
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