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"""
Support for prepared statements
"""
# Copyright (C) 2020 The Psycopg Team
from enum import IntEnum, auto
from typing import Iterator, Optional, Sequence, Tuple, TYPE_CHECKING
from collections import OrderedDict
from typing_extensions import TypeAlias
from . import pq
from ._compat import Deque
from ._queries import PostgresQuery
if TYPE_CHECKING:
from .pq.abc import PGresult
Key: TypeAlias = Tuple[bytes, Tuple[int, ...]]
COMMAND_OK = pq.ExecStatus.COMMAND_OK
TUPLES_OK = pq.ExecStatus.TUPLES_OK
class Prepare(IntEnum):
NO = auto()
YES = auto()
SHOULD = auto()
class PrepareManager:
# Number of times a query is executed before it is prepared.
prepare_threshold: Optional[int] = 5
# Maximum number of prepared statements on the connection.
prepared_max: int = 100
def __init__(self) -> None:
# Map (query, types) to the number of times the query was seen.
self._counts: OrderedDict[Key, int] = OrderedDict()
# Map (query, types) to the name of the statement if prepared.
self._names: OrderedDict[Key, bytes] = OrderedDict()
# Counter to generate prepared statements names
self._prepared_idx = 0
self._maint_commands = Deque[bytes]()
@staticmethod
def key(query: PostgresQuery) -> Key:
return (query.query, query.types)
def get(
self, query: PostgresQuery, prepare: Optional[bool] = None
) -> Tuple[Prepare, bytes]:
"""
Check if a query is prepared, tell back whether to prepare it.
"""
if prepare is False or self.prepare_threshold is None:
# The user doesn't want this query to be prepared
return Prepare.NO, b""
key = self.key(query)
name = self._names.get(key)
if name:
# The query was already prepared in this session
return Prepare.YES, name
count = self._counts.get(key, 0)
if count >= self.prepare_threshold or prepare:
# The query has been executed enough times and needs to be prepared
name = f"_pg3_{self._prepared_idx}".encode()
self._prepared_idx += 1
return Prepare.SHOULD, name
else:
# The query is not to be prepared yet
return Prepare.NO, b""
def _should_discard(self, prep: Prepare, results: Sequence["PGresult"]) -> bool:
"""Check if we need to discard our entire state: it should happen on
rollback or on dropping objects, because the same object may get
recreated and postgres would fail internal lookups.
"""
if self._names or prep == Prepare.SHOULD:
for result in results:
if result.status != COMMAND_OK:
continue
cmdstat = result.command_status
if cmdstat and (cmdstat.startswith(b"DROP ") or cmdstat == b"ROLLBACK"):
return self.clear()
return False
@staticmethod
def _check_results(results: Sequence["PGresult"]) -> bool:
"""Return False if 'results' are invalid for prepared statement cache."""
if len(results) != 1:
# We cannot prepare a multiple statement
return False
status = results[0].status
if COMMAND_OK != status != TUPLES_OK:
# We don't prepare failed queries or other weird results
return False
return True
def _rotate(self) -> None:
"""Evict an old value from the cache.
If it was prepared, deallocate it. Do it only once: if the cache was
resized, deallocate gradually.
"""
if len(self._counts) > self.prepared_max:
self._counts.popitem(last=False)
if len(self._names) > self.prepared_max:
name = self._names.popitem(last=False)[1]
self._maint_commands.append(b"DEALLOCATE " + name)
def maybe_add_to_cache(
self, query: PostgresQuery, prep: Prepare, name: bytes
) -> Optional[Key]:
"""Handle 'query' for possible addition to the cache.
If a new entry has been added, return its key. Return None otherwise
(meaning the query is already in cache or cache is not enabled).
"""
# don't do anything if prepared statements are disabled
if self.prepare_threshold is None:
return None
key = self.key(query)
if key in self._counts:
if prep is Prepare.SHOULD:
del self._counts[key]
self._names[key] = name
else:
self._counts[key] += 1
self._counts.move_to_end(key)
return None
elif key in self._names:
self._names.move_to_end(key)
return None
else:
if prep is Prepare.SHOULD:
self._names[key] = name
else:
self._counts[key] = 1
return key
def validate(
self,
key: Key,
prep: Prepare,
name: bytes,
results: Sequence["PGresult"],
) -> None:
"""Validate cached entry with 'key' by checking query 'results'.
Possibly record a command to perform maintenance on database side.
"""
if self._should_discard(prep, results):
return
if not self._check_results(results):
self._names.pop(key, None)
self._counts.pop(key, None)
else:
self._rotate()
def clear(self) -> bool:
"""Clear the cache of the maintenance commands.
Clear the internal state and prepare a command to clear the state of
the server.
"""
self._counts.clear()
if self._names:
self._names.clear()
self._maint_commands.clear()
self._maint_commands.append(b"DEALLOCATE ALL")
return True
else:
return False
def get_maintenance_commands(self) -> Iterator[bytes]:
"""
Iterate over the commands needed to align the server state to our state
"""
while self._maint_commands:
yield self._maint_commands.popleft()