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card_est.py
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# Copyright 2022 The Balsa Authors.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
from balsa.util import postgres
class CardEst(object):
"""Base class for cardinality estimators."""
def __call__(self, node, join_conds):
raise NotImplementedError()
class PostgresCardEst(CardEst):
def __init__(self):
self._cache = {}
def _HashKey(self, node):
"""Computes a hash key based on the logical contents of 'node'.
Specifically, hash on the sorted sets of table IDs and their filters.
NOTE: Postgres can produce slightly different cardinality estimates
when all being equal but just the FROM list ordering tables
differently. Here, we ignore this slight difference.
"""
sorted_filters = '\n'.join(sorted(node.GetFilters()))
sorted_leaves = '\n'.join(sorted(node.leaf_ids()))
return sorted_leaves + sorted_filters
def __call__(self, node, join_conds):
key = self._HashKey(node)
card = self._cache.get(key)
if card is None:
sql_str = node.to_sql(join_conds)
card = postgres.GetCardinalityEstimateFromPg(sql=sql_str)
self._cache[key] = card
return card