An index can seek by a subset of characters, as long as you're searching from the left. E.g., "Inter%" can seek, "%net" will not.
However, the first character is not necessarily the character under which the article would be sorted. "The Internet" should go under "I", not "T". You probably need two fields, DisplayTitle
and SortTitle
; a single-character index on the latter may be worthwhile, but most likely a full-length index will be just fine.
Indexes are typically B-trees, and a seek will jump to the right location about equally quickly whether you have 10 or 100 entries per page. Scans are another matter, but I'd start with the simplest solution and add an extra index only if performance proves inadequate in practice.
QUESTION #1
Why are there different levels of MySQL collation/charsets?
ANSWER TO QUESTION #1
There are two good reasons for different character sets and collations
Reason #1 : Disk Space
When you run this query
SELECT
maxlen,
GROUP_CONCAT(CHARACTER_SET_NAME) CharSets,
COUNT(1) CharSetCount
FROM information_schema.character_sets
GROUP BY maxlen\G
You get this:
mysql> SELECT
-> maxlen,
-> GROUP_CONCAT(CHARACTER_SET_NAME) CharSets,
-> COUNT(1) CharSetCount
-> FROM information_schema.character_sets
-> GROUP BY maxlen\G
*************************** 1. row ***************************
maxlen: 1
CharSets: cp1257,cp850,binary,koi8r,latin2,ascii,tis620,koi8u,greek,armscii8,keybcs2,macroman,latin7,cp1251,cp1256,dec8,hp8,geostd8,latin1,swe7,hebrew,cp1250,latin5,cp866,macce,cp852
CharSetCount: 26
*************************** 2. row ***************************
maxlen: 2
CharSets: big5,cp932,sjis,gbk,ucs2,euckr,gb2312
CharSetCount: 7
*************************** 3. row ***************************
maxlen: 3
CharSets: eucjpms,ujis,utf8
CharSetCount: 3
*************************** 4. row ***************************
maxlen: 4
CharSets: utf16,utf32,utf8mb4
CharSetCount: 3
4 rows in set (0.00 sec)
mysql>
Some character sets have a Maximum Length of 1 byte to represent a character. Other need more. Give this information, you may want to refrain from using the eucjpms, ujis, utf8, utf16, utf32, utf8mb4 character sets so that VARCHAR and TEXT data takes less space on disk.
Reason #2 : Internationalization
Characters Sets Each Come With One or More Collations to cover a variety of Languages
When you run this query
SELECT
A.CHARACTER_SET_NAME,
GROUP_CONCAT(COLLATION_NAME) Collations,
COUNT(1) CollationCount
FROM
information_schema.character_sets A
INNER JOIN information_schema.collations B
USING (CHARACTER_SET_NAME)
GROUP BY A.CHARACTER_SET_NAME\G
You will see that some Characters Sets have with multiple collations for Different Parts of Europe. Chinese, Japanese, Greek, and parts of Asia Minor and Scandinavia are also available.
QUESTION #2
Should you always ensure your PHP connection matches the charset of the database you're working on?
ANSWER TO QUESTION #2
SCENARIO
You are driving at 3:00 AM. You are the only driver on the road. You come to an intersection. You have the red light.
Question : Do you stop or go through the red light?
Answer : Depends on the neighborhood
- Safe neighborhood ?
- Some abide by the law, stop at the red, and wait for green.
- Some chance it and go through
- Bad neighborhood or new to the area ?
- Some abide by the law, stop at the red, and wait for green AT THE RISK OF A CARJACKING
- Some chance it and go through to AVOID OR REDUCE RISK OF A CARJACKING
- Assume the worst and find another route
How does this apply?
You should err on the side of caution. You should always check the charset beforehand because you do not know the neighborhood (client program, internet browser) the PHP connection will be entering and if there is a risk of a carjacking (putting invalid data into the database, requesting too much data for retrieval).
QUESTION #3
If you can have different tables that use different character sets do you just use SET NAMES or mysql(i)_set_charset to switch?
ANSWER TO QUESTION #3
By all means
QUESTION #4
If you have a table that has multiple charsets how do you manage that since the connection can only use one charset at a time?
ANSWER TO QUESTION #4
You may have to shift character sets with the DB Session. Here are the settings that can be changed at the session level:
Please set these carefully before reading from and writing to the database. It would also be wise to store the character set name and collation in the same table you will be accessing.
Best Answer
Because MySQL is being nice and auto-correcting a user error, but only because it can.
You are not wrong that:
and:
However, you are not seeing what you think you are seeing in your test because you are not testing with data that requires the "mb4" version of the "utf8" collations. You are testing with a "safe" value that fits into the non-supplementary character-capable
utf8
charset. In this situation, MySQL simply uses the charset of the column —utf8
— and theutf8_bin
collation is just fine.The world looks a little different when you use a 4 byte character (i.e. a supplementary character). In this case, the character cannot exist in the
utf8
charset (because theutf8
charset can only handle BMP characters / the first 65,536 code points / U+0000 through U+FFFF / characters that are 1 to 3 bytes each). And so, since the charset of the string literal cannot be changed to the one that does work with the column's collation —utf8_bin
— you would get an error stating:I tested with code point U+1F369 (i.e. the donut emoji: "?"):
See the above example code in action on dbfiddle.uk
The result set from the first
SELECT
is:which shows that the
utf8mb4
string was coerced intoutf8
, and everyone was happy. Was happy, until Mr Donut showed up in the secondSELECT
statement (you would think that donuts would make everything better, though, wouldn't you? I generally do ?).