sqlglot.optimizer.pushdown_projections
1from collections import defaultdict 2 3from sqlglot import alias, exp 4from sqlglot.helper import flatten 5from sqlglot.optimizer.qualify_columns import Resolver 6from sqlglot.optimizer.scope import Scope, traverse_scope 7from sqlglot.schema import ensure_schema 8 9# Sentinel value that means an outer query selecting ALL columns 10SELECT_ALL = object() 11 12# Selection to use if selection list is empty 13DEFAULT_SELECTION = lambda: alias("1", "_") 14 15 16def pushdown_projections(expression, schema=None): 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 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 _remove_unused_selections(scope, parent_selections, schema) 61 62 # Group columns by source name 63 selects = defaultdict(set) 64 for col in scope.columns: 65 table_name = col.table 66 col_name = col.name 67 selects[table_name].add(col_name) 68 69 # Push the selected columns down to the next scope 70 for name, (_, source) in scope.selected_sources.items(): 71 if isinstance(source, Scope): 72 columns = selects.get(name) or set() 73 referenced_columns[source].update(columns) 74 75 return expression 76 77 78def _remove_unused_selections(scope, parent_selections, schema): 79 order = scope.expression.args.get("order") 80 81 if order: 82 # Assume columns without a qualified table are references to output columns 83 order_refs = {c.name for c in order.find_all(exp.Column) if not c.table} 84 else: 85 order_refs = set() 86 87 new_selections = defaultdict(list) 88 removed = False 89 star = False 90 for selection in scope.selects: 91 name = selection.alias_or_name 92 93 if SELECT_ALL in parent_selections or name in parent_selections or name in order_refs: 94 new_selections[name].append(selection) 95 else: 96 if selection.is_star: 97 star = True 98 removed = True 99 100 if star: 101 resolver = Resolver(scope, schema) 102 103 for name in sorted(parent_selections): 104 if name not in new_selections: 105 new_selections[name].append( 106 alias(exp.column(name, table=resolver.get_table(name)), name) 107 ) 108 109 # If there are no remaining selections, just select a single constant 110 if not new_selections: 111 new_selections[""].append(DEFAULT_SELECTION()) 112 113 scope.expression.select(*flatten(new_selections.values()), append=False, copy=False) 114 115 if removed: 116 scope.clear_cache()
def
DEFAULT_SELECTION():
14DEFAULT_SELECTION = lambda: alias("1", "_")
def
pushdown_projections(expression, schema=None):
17def pushdown_projections(expression, schema=None): 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 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 _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
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
Returns:
sqlglot.Expression: optimized expression