Add markdown lock trace report

[kaaveland]

Add a first draft markdown report of a lock trace, something suitable to post as a comment on a merge request. This PR is big because it also deletes the machinery that was in place for supporting multiple formats and detail levels, in favor of doing something simpler -- just working with strings. Here's the example report:

Lock trace of mig.sql

In the below trace, a lock is considered dangerous if it conflicts with application code queries, such as SELECT or INSERT.

We should aim to hold dangerous locks for as little time as possible. If a dangerous lock is held while doing an operation that does not require it, we should split the migration into two steps.

For example, it often will make sense to add a new column in one migration, then backfill it in a separate one, since adding a column requires an AccessExclusiveLock while backfilling can do with a RowExclusiveLock which is much less likely to block concurrent transactions.

Overall Summary

Started at	Total duration (ms)	Number of dangerous locks
2024-05-05T00:45:47.027917+02:00	1037	1 ❌

All locks acquired

Schema	Object	Mode	Relkind	OID	Safe	Duration held (ms)
public	books	AccessExclusiveLock	Table	16386	❌	1011

Dangerous locks explanation

• AccessExclusiveLock would block the following operations on public.books:

  • SELECT
  • FOR UPDATE
  • FOR NO KEY UPDATE
  • FOR SHARE
  • FOR KEY SHARE
  • UPDATE
  • DELETE
  • INSERT
  • MERGE

Statement number 1 for 26 ms

SQL

create table author(name text not null);

Locks at start

No locks held at the start of this statement.

New locks taken

No new locks taken by this statement.

Statement number 2 for 1 ms

SQL

alter table books alter column title set not null;

Locks at start

No locks held at the start of this statement.

New locks taken

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessExclusiveLock	Table	16386	❌

Statement number 3 for 1007 ms

SQL

select pg_sleep(1);

Locks at start

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessExclusiveLock	Table	16386	❌

New locks taken

No new locks taken by this statement.

Statement number 4 for 3 ms

SQL

select * from books;

Locks at start

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessExclusiveLock	Table	16386	❌

New locks taken

No new locks taken by this statement.

[kaaveland]

Eugene 🔒 trace of add_authors.sql

In the below trace, a lock is considered dangerous if it conflicts with application code queries, such as SELECT or INSERT.

We should aim to hold dangerous locks for as little time as possible. If a dangerous lock is held while doing an operation that does not require it, we should split the migration into two steps.

For example, it often will make sense to add a new column in one migration, then backfill it in a separate one, since adding a column requires an AccessExclusiveLock while backfilling can do with a RowExclusiveLock which is much less likely to block concurrent transactions.

Overall Summary

Started at	Total duration (ms)	Number of dangerous locks
2024-05-04T23:18:19.997774940+00:00	1002	1 ❌

All locks acquired

Schema	Object	Mode	Relkind	OID	Safe	Duration held (ms)
public	books	AccessExclusiveLock	Table	16389	❌	1001

Dangerous locks found

• AccessExclusiveLock would block the following operations on public.books:
  • SELECT + FOR UPDATE + FOR NO KEY UPDATE + FOR SHARE + FOR KEY SHARE + UPDATE + DELETE + INSERT + MERGE

Statement number 1 for 1 ms

SQL

create table authors(name text not null);

Locks at start

No locks held at the start of this statement.

New locks taken

No new locks taken by this statement.

Statement number 2 for 0 ms

SQL

alter table books alter column title set not null;

Locks at start

No locks held at the start of this statement.

New locks taken

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessExclusiveLock	Table	16389	❌

Statement number 3 for 1001 ms

SQL

select pg_sleep(1);

Locks at start

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessExclusiveLock	Table	16389	❌

New locks taken

No new locks taken by this statement.

Statement number 4 for 0 ms

SQL

select * from books;

Locks at start

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessExclusiveLock	Table	16389	❌

New locks taken

No new locks taken by this statement.

[kaaveland]

Eugene 🔒 trace of add_authors.sql

In the below trace, a lock is considered dangerous if it conflicts with application code queries, such as SELECT or INSERT.

We should aim to hold dangerous locks for as little time as possible. If a dangerous lock is held while doing an operation that does not require it, we should split the migration into two steps.

For example, it often will make sense to add a new column in one migration, then backfill it in a separate one, since adding a column requires an AccessExclusiveLock while backfilling can do with a RowExclusiveLock which is much less likely to block concurrent transactions.

Overall Summary

Started at	Total duration (ms)	Number of dangerous locks
2024-05-04T23:20:37.765772580+00:00	1003	1 ❌

All locks acquired

Schema	Object	Mode	Relkind	OID	Safe	Duration held (ms)
public	books	AccessExclusiveLock	Table	16389	❌	1001
public	books	AccessShareLock	Table	16389	✅	0
public	books_pkey	AccessShareLock	Index	16395	✅	0

Dangerous locks found

• AccessExclusiveLock would block the following operations on public.books:
  • SELECT
  • FOR UPDATE
  • FOR NO KEY UPDATE
  • FOR SHARE
  • FOR KEY SHARE
  • UPDATE
  • DELETE
  • INSERT
  • MERGE

Statement number 1 for 2 ms

SQL

create table authors(name text not null);

Locks at start

No locks held at the start of this statement.

New locks taken

No new locks taken by this statement.

Statement number 2 for 0 ms

SQL

alter table books alter column title set not null;

Locks at start

No locks held at the start of this statement.

New locks taken

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessExclusiveLock	Table	16389	❌

Statement number 3 for 1001 ms

SQL

select pg_sleep(1);

Locks at start

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessExclusiveLock	Table	16389	❌

New locks taken

No new locks taken by this statement.

Statement number 4 for 0 ms

SQL

select * from books;

Locks at start

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessExclusiveLock	Table	16389	❌

New locks taken

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessShareLock	Table	16389	✅
public	books_pkey	AccessShareLock	Index	16395	✅

[kaaveland]

Eugene 🔒 trace of add_authors.sql

In the below trace, a lock is considered dangerous if it conflicts with application code queries, such as SELECT or INSERT.

We should aim to hold dangerous locks for as little time as possible. If a dangerous lock is held while doing an operation that does not require it, we should split the migration into two steps.

For example, it often will make sense to add a new column in one migration, then backfill it in a separate one, since adding a column requires an AccessExclusiveLock while backfilling can do with a RowExclusiveLock which is much less likely to block concurrent transactions.

Overall Summary

Started at	Total duration (ms)	Number of dangerous locks
2024-05-04T23:25:16.715110214+00:00	1002	1 ❌

All locks acquired

Schema	Object	Mode	Relkind	OID	Safe	Duration held (ms)
public	books	AccessExclusiveLock	Table	16389	❌	1001
public	books_pkey	AccessShareLock	Index	16395	✅	0
public	books	AccessShareLock	Table	16389	✅	0

Dangerous locks found

• AccessExclusiveLock would block the following operations on public.books:
  • SELECT
  • FOR UPDATE
  • FOR NO KEY UPDATE
  • FOR SHARE
  • FOR KEY SHARE
  • UPDATE
  • DELETE
  • INSERT
  • MERGE

Statement number 1 for 1 ms

SQL

create table authors(name text not null);

Locks at start

No locks held at the start of this statement.

New locks taken

No new locks taken by this statement.

Statement number 2 for 0 ms

SQL

alter table books alter column title set not null;

Locks at start

No locks held at the start of this statement.

New locks taken

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessExclusiveLock	Table	16389	❌

Statement number 3 for 1001 ms

SQL

select pg_sleep(1);

Locks at start

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessExclusiveLock	Table	16389	❌

New locks taken

No new locks taken by this statement.

Statement number 4 for 0 ms

SQL

select * from books;

Locks at start

Schema	Object	Mode	Relkind	OID	Safe
public	books	AccessExclusiveLock	Table	16389	❌

New locks taken

Schema	Object	Mode	Relkind	OID	Safe
public	books_pkey	AccessShareLock	Index	16395	✅
public	books	AccessShareLock	Table	16389	✅