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Query Plan SQL construct
Michael Bayer edited this page Nov 18, 2024
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This example illustrates how to implement PostgreSQL's EXPLAIN and EXPLAIN ANALYZE around SQLAlchemy Core statement objects, using a construct that can be executed like any other statement to provide the results of a query plan. Syntaxes for other databases can be added as well.
Examples for 1.4 and 1.3 are shown below. Note the 1.4 interface for creating a custom SQL object might be simplified a bit soon.
"""EXPLAIN any query.
Includes PostgreSQL EXPLAIN syntax.
SQLAlchemy 1.4 / 2.0 version.
"""
from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import Integer
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.sql.expression import ClauseElement
from sqlalchemy.sql.expression import Executable
class explain(Executable, ClauseElement):
inherit_cache = False
def __init__(self, stmt, analyze=False):
self.statement = stmt
self.analyze = analyze
@compiles(explain, "postgresql")
def pg_explain(element, compiler, **kw):
text = "EXPLAIN "
if element.analyze:
text += "ANALYZE "
text += compiler.process(element.statement, **kw)
return text
if __name__ == "__main__":
from sqlalchemy.orm import aliased
from sqlalchemy.orm import declarative_base
from sqlalchemy.orm import Session
from sqlalchemy import select
Base = declarative_base()
class Foo(Base):
__tablename__ = "foo"
id = Column(Integer, primary_key=True)
engine = create_engine(
"postgresql://scott:tiger@localhost/test", echo=True
)
Base.metadata.create_all(engine)
with Session(engine) as sess:
falias = aliased(Foo)
q = select(Foo).where(Foo.id > 5).join(falias, Foo.id == falias.id)
print(sess.execute(explain(q)).fetchall())"""EXPLAIN any query.
Includes PostgreSQL EXPLAIN syntax.
SQLAlchemy 1.3 and prior version.
"""
from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import Integer
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.sql.expression import _literal_as_text
from sqlalchemy.sql.expression import ClauseElement
from sqlalchemy.sql.expression import Executable
class explain(Executable, ClauseElement):
def __init__(self, stmt, analyze=False):
self.statement = _literal_as_text(stmt)
self.analyze = analyze
@compiles(explain, "postgresql")
def pg_explain(element, compiler, **kw):
text = "EXPLAIN "
if element.analyze:
text += "ANALYZE "
text += compiler.process(element.statement, **kw)
return text
if __name__ == "__main__":
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import Session
from sqlalchemy.orm import aliased
Base = declarative_base()
class Foo(Base):
__tablename__ = "foo"
id = Column(Integer, primary_key=True)
engine = create_engine(
"postgresql://scott:tiger@localhost/test", echo=True
)
Base.metadata.create_all(engine)
sess = Session(engine)
falias = aliased(Foo)
q = sess.query(Foo).filter(Foo.id > 5).join(falias, Foo.id == falias.id)
print(sess.execute(explain(q)).fetchall())