Coverage for python / lsst / daf / butler / registry / interfaces / _database_explain.py: 48%

1# This file is part of daf_butler. 

2# 

3# Developed for the LSST Data Management System. 

4# This product includes software developed by the LSST Project 

5# (http://www.lsst.org). 

6# See the COPYRIGHT file at the top-level directory of this distribution 

7# for details of code ownership. 

8# 

9# This software is dual licensed under the GNU General Public License and also 

10# under a 3-clause BSD license. Recipients may choose which of these licenses 

11# to use; please see the files gpl-3.0.txt and/or bsd_license.txt, 

12# respectively. If you choose the GPL option then the following text applies 

13# (but note that there is still no warranty even if you opt for BSD instead): 

14# 

15# This program is free software: you can redistribute it and/or modify 

16# it under the terms of the GNU General Public License as published by 

17# the Free Software Foundation, either version 3 of the License, or 

18# (at your option) any later version. 

19# 

20# This program is distributed in the hope that it will be useful, 

21# but WITHOUT ANY WARRANTY; without even the implied warranty of 

22# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 

23# GNU General Public License for more details. 

24# 

25# You should have received a copy of the GNU General Public License 

26# along with this program. If not, see <http://www.gnu.org/licenses/>. 

27 

28from __future__ import annotations 

29 

30from typing import Any 

31 

32from sqlalchemy import ClauseElement, Connection, Executable 

33from sqlalchemy.ext.compiler import compiles 

34from sqlalchemy.sql.compiler import SQLCompiler 

35 

36 

37def get_query_plan(connection: Connection, sql: ClauseElement) -> str: 

38 """Retrieve the query plan for a given statement from the DB as a 

39 human-readable string. 

40 

41 Parameters 

42 ---------- 

43 connection : `sqlalchemy.Connection` 

44 Database connection used to retrieve query plan. 

45 sql : `sqlalchemy.ClauseElement` 

46 SQL statement for which we will retrieve a query plan. 

47 """ 

48 if connection.dialect.name != "postgresql": 

49 # This could be implemented for SQLite using its EXPLAIN QUERY PLAN 

50 # syntax, but the result rows are a little different and we haven't had 

51 # a need for it yet. 

52 return "(not available)" 

53 

54 with connection.execute(_Explain(sql)) as explain_cursor: 

55 lines = explain_cursor.scalars().all() 

56 return "\n".join(lines) 

57 

58 

59# This is based on code from the sqlalchemy wiki at 

60# https://github.com/sqlalchemy/sqlalchemy/wiki/Query-Plan-SQL-construct 

61class _Explain(Executable, ClauseElement): 

62 """Custom SQLAlchemy construct for retrieving query plan from the DB. 

63 

64 Parameters 

65 ---------- 

66 statement : `sqlalchemy.ClauseElement` 

67 SQLAlchemy SELECT statement to retrieve query plan for. 

68 """ 

69 

70 def __init__(self, statement: ClauseElement) -> None: 

71 self.statement = statement 

72 

73 

74@compiles(_Explain, "postgresql") 

75def _compile_explain(element: _Explain, compiler: SQLCompiler, **kw: Any) -> str: 

76 text = "EXPLAIN (VERBOSE TRUE, SETTINGS TRUE)" 

77 text += compiler.process(element.statement, **kw) 

78 

79 return text