A Little Noise

August 24, 2015

Swap Endian

Filed under: MySQL — snoyes @ 12:34 pm
-- Expects a hex string: AbCdEf
-- Returns the string swapped for endianness: EfCdAb


  WHILE position < LENGTH(inString) DO
    SET holder = CONCAT(SUBSTRING(inString, position, 2), holder);
    SET position = position + 2;

  RETURN holder;

So you can do things like:

          ), 1, 8
      ), 16, 10)
  ) AS event_timestamp;

April 22, 2015

Breakpoints for stored procedures and functions

Filed under: MySQL — snoyes @ 5:47 pm

and without creating a table to pass the state around (really just an excuse to use the named locks feature).

RETURNS tinyint
	-- Acquire lock 1
	-- Wait until lock 2 is taken to signal that we may continue
		DO 1;

	-- Acquire lock 3 to acknowledge message to continue.
	-- Wait for lock 2 to be released as signal of receipt.
		DO 1;


RETURNS tinyint
	-- Acquire lock 2 as a signal to go past the breakpoint
	-- Wait until lock 3 is taken as signal of receipt.
	DO GET_LOCK(CONCAT('lock_2_', connection_id), -1);
		DO 1;
	UNTIL IS_USED_LOCK(CONCAT('lock_3_', connection_id)) END REPEAT;
	DO RELEASE_LOCK(CONCAT('lock_2_', connection_id));


DROP PROCEDURE IF EXISTS test_the_breakpoints//
CREATE PROCEDURE test_the_breakpoints()
	SELECT CONCAT('In another session: DO NEXT_BREAKPOINT(', CONNECTION_ID(), ');') as `instructions`;


	SELECT 'do it again' as `now:`;


	SELECT 'end' as `the`;

CALL test_the_breakpoints();

March 7, 2015

log event entry exceeded max_allowed_packet

Filed under: MySQL — snoyes @ 1:07 pm

Sometimes replication halts with an error like:

Slave I/O: Got fatal error 1236 from master when reading data from binary log
Error reading packet from server: log event entry exceeded max_allowed_packet; Increase max_allowed_packet on master;

If it’s the SQL thread instead of the I/O thread, it might complain about ‘Event too big’. The error could also be the other direction, complaining of ‘Event too small’.

I rarely see this error actually have anything to do with max_allowed_packet. You can check the largest event in the master’s binary log file. Even though the binary log records the length in each entry, mysqlbinlog doesn’t expose it, so we have to do the math ourselves:

mysqlbinlog mysql-bin.00XXX | gawk "/^# at / { diff = $3 - prev; prev = $3; } (diff > max) { max = diff } END {print max}" -

If the result is larger than max_allowed_packet, then the problem and solution are exactly what the the error message says. If it’s smaller, or if that also gives you an error about the length, then read on.

Assuming that’s not the problem, here’s what may be happening:

After the 4-byte magic number at the very beginning of the binary log, every entry in the binary log (regardless of the binlog_format) starts with a 19-byte header.

  1. 4 bytes for the timestamp
  2. 1 byte for the event type
  3. 4 bytes for the server_id that wrote the event
  4. 4 bytes for the length of the event (including the header)
  5. 4 bytes for the position where the next event starts (which should be the position where this event starts plus the length of this event)
  6. 2 bytes for some flags that are usually set to 0.

These are all written little-endian. An example:

123e d254 0201 0000 005b 0000 0083 9f3c 3300 00…

4 byte timestamp: 123e d254 = 1423064594 = 2015-02-04 09:43:14 CST
1 byte type code: 02 = QUERY_EVENT
4 byte server id: 01 0000 00 = 1
4 byte length: 5b 0000 00 = 91 bytes
4 byte next pos: 83 9f3c 33 = 859,611,011

Replication works like this:

  1. the master executes a statement and records the statement or resulting row events in its binary log
  2. the slave’s IO thread says to the master, “I’m at position X. What’s next?”
  3. the master reads the binary log and passes the statement or row events to the slave
  4. the slave’s IO thread writes what it got from the master into its relay log
  5. the slave’s SQL thread reads the relay log and executes the statement or row events

Occasionally, something goes wrong somewhere in that chain:

  • Sometimes a piece of the event never makes it into the binary log (the header says “I contain 100 bytes”, but then only 90 bytes appear in the file before the next event begins).
  • Sometimes the slave asks the master for the wrong position (someone has been mucking about with CHANGE MASTER TO and using the wrong numbers?)
  • Sometimes the event doesn’t make it across the wire from the master to the slave in one piece (network problems?)

However it happens, we end up in the wrong place in the stream of bits, usually somewhere in the middle of the next event’s data section. The slave doesn’t know it’s in the wrong place (there are no sign posts or magic numbers to identify an event), so it just reads the next 19 bytes it sees as if they were the event header. Let’s see what happens when just one byte has gone missing:

123e d254 0201 0000 005b 0000 0083 9f3c 3300

4 byte timestamp: 3ed2 5402 = 39113278 = 1971-03-29 11:47:58 CST
1 byte type code: 01 = START_EVENT_V3
4 byte server id: 0000 005b = 1526726656
4 byte length: 0000 0083 = 2,197,815,296 bytes
4 byte next pos: 9f3c 3300 = 3,357,855

So the slave gets this header and thinks the event length is over 2GB, which is double the highest possible max_allowed_packet, so it throws an error. The real problem isn’t that the event is too big, but that it’s reading the wrong place and interpreting the wrong data as a length.

It would be nice if it would do a sanity check and notice that the current position plus the length doesn’t equal the next position and produce a more helpful error message.

From MySQL 5.6.6 on, by default a checksum is written at the end of each event in the log, which would also catch most of these kinds of errors. But neither the master nor the slave examine those checksums by default. Consider enabling –master-verify-checksum on the master and –slave-sql-verify-checksum on the slave.

How do you fix it?

First check if the master’s binary log is OK. Just run it through mysqlbinlog and redirect the output to NUL or /dev/null. If it makes it through the whole thing with no errors, then the problem is just with the slave’s relay log or file position. If you’re not using GTIDs, then use CHANGE MASTER TO and give it the Exec_Master_Log_Pos from SHOW SLAVE STATUS. It will discard the old relay logs and copy them fresh from the master. Hopefully this time it will get the whole event properly and the problem will go away. If you are using GTIDs, then a RESET SLAVE will discard the relay logs, and when you START SLAVE it will figure out on its own what it needs from the master. If you have to do this frequently, check your network.

If the master’s binary log produces errors when you run it through mysqlbinlog, recovery is not so easy. There have been a few bugs where the master doesn’t record the whole event properly, often having to do with binlogs that exceed 4GB in size. You (or the people you pay for MySQL support) might be able to examine the binary log file, figure out exactly what statement has been corrupted, run it directly on the slave, find the position where the next good event begins, and continue from there. If you can’t recover that statement but you know the tables it affected, you might be able to just copy those few tables from the master to the slave, and have replication ignore them until it catches up to the time of the copy. Or you can use some utility to compare the two servers and fix any differences. Or you can copy all the data from the master and set up the slave again as if it was brand new.

Whatever you do, don’t just set SQL_SLAVE_SKIP_COUNTER=1 or slave_skip_errors and hope the problem goes away. It will only make it worse.

November 6, 2014

Undelete rows with the binary log

Filed under: MySQL — snoyes @ 3:18 pm

Can you use the binary logs to undo a DELETE? Maybe, if you’re using ROW format logging. The difference between a delete and an insert event could be just one byte – the one that maps it as a DELETE_ROWS_EVENT or a WRITE_ROWS_EVENT. Let’s try it.

I’ve already populated this table with a few rows:

CREATE TABLE `undo_test` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`v` varchar(20) DEFAULT NULL,
`b` blob,

Save a hash so we can see if the undo really works later:

mysql -e "SELECT * FROM test.undo_test" | md5sum > before.md5

Delete an unwanted row:

DELETE FROM undo_test;
Query OK, 1693 rows affected (0.14 sec)

Oops! Forgot the WHERE clause! And of course I don’t have any recent backups. Is the data really gone?

mysql> select count(*) from undo_test;
| count(*) |
| 0 |

But maybe there’s hope. Find the delete event in the binary log.

| Log_name   | Pos    | Event_type  | Server_id | End_log_pos | Info
| bin.000001 |      4 | Format_desc |         1 |         120 | Server ver: 5.6.20-log, Binlog ver: 4
| bin.000001 |    516 | Query       |         1 |         588 | BEGIN
| bin.000001 |    588 | Table_map   |         1 |         647 | table_id: 70 (test.undo_test)
| bin.000001 |    647 | Delete_rows |         1 |        8859 | table_id: 70
| bin.000001 |   8859 | Delete_rows |         1 |       16932 | table_id: 70
| bin.000001 |  16932 | Delete_rows |         1 |       25040 | table_id: 70
| bin.000001 |  25040 | Delete_rows |         1 |       33244 | table_id: 70
| bin.000001 |  33244 | Delete_rows |         1 |       41408 | table_id: 70
| bin.000001 |  41408 | Delete_rows |         1 |       49549 | table_id: 70
| bin.000001 |  49549 | Delete_rows |         1 |       57764 | table_id: 70
| bin.000001 |  57764 | Delete_rows |         1 |       65956 | table_id: 70
| bin.000001 |  65956 | Delete_rows |         1 |       74096 | table_id: 70
| bin.000001 |  74096 | Delete_rows |         1 |       82237 | table_id: 70
| bin.000001 |  82237 | Delete_rows |         1 |       90445 | table_id: 70
| bin.000001 |  90445 | Delete_rows |         1 |       98586 | table_id: 70
| bin.000001 |  98586 | Delete_rows |         1 |      106728 | table_id: 70
| bin.000001 | 106728 | Delete_rows |         1 |      114886 | table_id: 70
| bin.000001 | 114886 | Delete_rows |         1 |      123085 | table_id: 70
| bin.000001 | 123085 | Delete_rows |         1 |      131290 | table_id: 70
| bin.000001 | 131290 | Delete_rows |         1 |      139476 | table_id: 70
| bin.000001 | 139476 | Delete_rows |         1 |      146945 | table_id: 70 flags: STMT_END_F
| bin.000001 | 146945 | Xid         |         1 |      146976 | COMMIT /* xid=36 */

The delete starts at position 516 and ends at 146976. We can pass those positions to mysqlbinlog to get just this delete event, along with the necessary header so mysql knows how to interpret the event.

From sql/log_event.h we see that a delete is 0x20 and an insert is 0x1E. This awk program will parse the mysqlbinlog output and make the necessary changes.


# In a ROW event, the first 9 bytes are timestamp (4 bytes), event type (1 byte), server id (4 bytes).
# In base64 encoding, 6 bits map to a single character.
# So the first 30 bits of the timestamp are the first 5 characters
# The last 2 bits of the timestamp and the first nibble of event type are character 6
# The last nibble of event type plus first 2 bits of server id are character 7
# The last 30 bits of server id are characters 8-12.

# A DELETE event will match the timestamp and sever id of the table map event (first line after BINLOG ') (I hope)
# Characters 6 and 7 have only four possible values each:
# Char6 is (00, 01, 10, or 11) followed by 0010. These map to CSiy
# Char7 is 0000 followed by (00, 01, 10, or 11). These map to ABCD

# To change DELETE to INSERT:
# - change the last 4 bits of character 6 to 0001. These map to BRhx.
# - change the first 4 bits of character 7 to 1110. These map to 4567.

BEGIN { regexp = "$^" }

  regexp = "^" substr($0, 1, 5) "[CSiy][ABCD]" substr($0, 8, 4);

$0 ~ regexp {
  n = $0;
  $0 = substr(n, 1, 5)
  $0 = $0 substr("BRhx", index("CSiy", substr(n, 6, 1)), 1)
  $0 = $0 substr("4567", index("ABCD", substr(n, 7, 1)), 1)
  $0 = $0 substr(n, 8) ;

{ print; }

Pass the event through that code, and then back into mysql:

mysqlbinlog --start-position=516 --stop-position=146976 bin.000001 | awk -f undelete.awk | mysql

Compare it to the original hash to see if it’s really back:

mysql -e "SELECT * FROM test.undo_test" | md5sum -c before.md5
-: OK

Looks like it worked.

October 1, 2014

Ambiguous column names (not that kind)

Filed under: MySQL Gotchas — snoyes @ 3:18 pm

Time for another quiz to impress your friends, if you have that kind of friends.

The manual page in question

mysql --skip-enable-disable-column-names=0 -e "select 1"
mysql --skip-disable-enable-column-names=0 -e "select 1"
mysql --loose-disable-enable-skip-column-names=0 -e "select 1"

What’s the result? An error about unknown options? A warning? Column headers enabled? Disabled?

Answer ▼

April 29, 2014

Mental Activity

Filed under: Uncategorized — Tags: — snoyes @ 11:02 am

the verbal quibbling in which great minds, for want of better occupation, frequently expended all their energies.

Pike, Luke Owen, M.A. “Woman and Political Power.” Popular Science May 1872. Google Books Web. 29 Apr. 2014.

Was he talking about metaphysical arguments from the middle ages, or YouTube comments?

April 28, 2014

Science and Immortality

Filed under: Uncategorized — Tags: — snoyes @ 11:35 am

A summary of the article “Science and Immortality“, by Rev. T. W. Fowle, Popular Science May 1872.

Modern science affects belief in two ways:

  1. Believers in science require proof from observable facts alone.
  2. Believers in science are biased against any supernatural explanation.

Four common arguments for belief in immortality:

  1. It is an “original intuition”, meaning we were created with the belief.
  2. It is a universal belief.
  3. It follows necessarily from the existence of God.
  4. It is essential as a motive for morality.


  1. It could have arisen after creation, and must have if Evolution is true as many men of science believe.
  2. It is not universally believed. Moses made no indication of believing in an afterlife, Buddha believed that eternity was non-existence, Julius Caesar believed in no-afterlife enough to consider death too good for his enemies.
  3. To a man of science demanding proof, the idea of God is just an attempt to explain our past which we don’t understand, and immortality is likewise an attempt to explain our future.
  4. Buddhists are moral, and immortality is not their motive. Pharaoh believed in immortality, and yet the resulting behavior was immoral. So it is neither necessary nor sufficient for morality.

It would be sufficient proof for life after death if a man were resurrected. Jesus was resurrected, and would be sufficient proof if the account were believed. Those who testified to the Resurrection may have been mistaken, but there is no reason to assume them to be liars. They were multiple men, who were able to see something amazing and miraculous and describe it in a sober and measured manner. We take historical testimony as factual all the time and rely upon it to be true. The only reason anyone doubts the testimony of the apostles is that it requires a supernatural element. When the miraculous is involved, then a certain turn of mind will simply assume that any testimony is always mistaken.

There is the Religious and, for lack of a better word, the “Rational,” at war within each of us, and we tend to one side or the other. Do we dwell on the present natural world or on hopes for the future? The rise of scientific thinking will cause doubts about the supernatural, but it will also help clear away erroneous dogmas. The bias to one side or the other will be created and sustained by moral means. Each side, religious and scientific, has moral value to offer.

There is no intellectual shortcut to the Christian faith; we cannot logically or scientifically prove the way to Christ (and here are my favorite quotes from this article: You can’t win by shouting). The conclusion is to live as Christ taught, and so win by faith what we cannot by force.

April 26, 2014

You can’t win by shouting

Filed under: Uncategorized — Tags: — snoyes @ 12:22 pm

…denunciations of moral turpitude only harden the hearts of men who ask for the bread of evidence and receive stones in the shape of insults.

…all attempts on the part of religion to confute the “skeptic” by purely intellectual methods are worse than useless. There is no intellectual short cut to the Christian faith…. It is not because men love the truth, but because they hate their enemies, that in things religious they desire to have what they call an overwhelming preponderance of argument on their side of the question, the possession of which enables them to treat their opponents as knaves or fools or both.

Fowle, Rev. T. W. “Science and Immortality.” Popular Science May 1872. Google Books Web. 26 Apr. 2014.

Fools think the government can fix everything.

Filed under: Uncategorized — Tags: — snoyes @ 8:44 am

Minds in which the conceptions of social actions are thus rudimentary, are also minds ready to harbor wild hopes of benefits to be achieved by administrative agencies. In each such mind there seems to be the unexpressed postulate that every evil in a society admits of cure; and that the cure lies within the reach of law.

[A superstitious conciousness] may even wonder how any being can reverence a thing shaped with his own hands; and yet it readily entertains subtler forms of the same feelings…. there is a tacit supposition that a government moulded by themselves has some efficiency beyond that naturally possessed by a certain group of citizens subsidized by the rest of the citizens.

…the instinct of self-preservation in each institution soon becomes dominant over every thing else; and maintains it when it performs some quite other function that that intended, or no function at all.

Spencer, Herbert. “The Study of Sociology.” Popular Science May 1872. Google Books Web. 26 Apr. 2014.

September 25, 2013

Pre-5.0.30 ADDTIME()

Filed under: MySQL — snoyes @ 1:25 pm

For when you want to add times greater than 839 hours.

CREATE FUNCTION ADDTIME_OLD(d datetime, t varchar(12))
RETURNS datetime
CONCAT(IF(LEFT(t, 1) = ‘-‘, ‘-‘, ”), ’00:’, SUBSTRING_INDEX(t, ‘:’, -2))

(Almost the same. I’m not going to figure out how to do the “you can use any delimiter you want” that MySQL supports.)

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