Good Day,
I have concern about indexing in mysql. I am trying to limit the Index size of specific DB table column which column names like ###ID.
this seems that the ID looks unique in first 8 bytes instead of entire length.
Thanks in advance.
As mysql documentation on creating indexes describes:
For string columns, indexes can be created that use only the leading part of column values, using col_name(length) syntax to specify an index prefix length.
Prefixes can be specified for CHAR, VARCHAR, BINARY, and VARBINARY column indexes.
Prefixes must be specified for BLOB and TEXT column indexes.
Prefix limits are measured in bytes, whereas the prefix length in CREATE TABLE, ALTER TABLE, and CREATE INDEX statements is interpreted
as number of characters for nonbinary string types (CHAR, VARCHAR,
TEXT) and number of bytes for binary string types (BINARY, VARBINARY,
BLOB). Take this into account when specifying a prefix length for a
nonbinary string column that uses a multibyte character set.
For spatial columns, prefix values cannot be given, as described later in this section.
The statement shown here creates an index using the first 10
characters of the name column (assuming that name has a nonbinary
string type):
CREATE INDEX part_of_name ON customer (name(10));
You can set the size after the field name. You must remove the old key first
ALTER TABLE mytab
ADD KEY `parent` (`parent`(8)) ;
Related
I'm using MySQL 5.6, I want to declare such type of DATATYPE in a Field during table creation which accepts only Alphabets not Number, For Example, If I have a Column (Name) in a table, then I want it to accept only alphabets from A to Z, not Number or Numeric Value, varchar, char etc.
the data type is not the place to enforce your data integrity. it's either chars or numbers, and numbers can be chars. If you really need to enforce it at the db level vs in code (eg php) then you must do a stored procedure
I am reading that MySQL 5.6 can only index the first 767 bytes of a varchar (or other text-based types). My schema character set is utf-8, so each character can be stored on up to 3 bytes. Since 767/3 = 255.66, this would indicate that the maximum length for a text column that needs to be indexed in 255 characters. Experience seems to confirm this as the following goes through:
create table gaga (
val varchar(255),
index(val)
) engine = InnoDB;
But changing the definition of val to varchar(256) yields an "Error Code: 1071. Specified key was too long; max key length is 767 bytes".
In this day in age, the limit to 255 characters seems awfully low, so: is this correct? If it is what is the best way to get larger pieces of text indexed with MySQL? (Should I avoid it? Store a SHA? Use another sort of index? Use another database character encoding?)
Though the limitation might seem ridiculous, it makes you think over if you really need the index for such a long varchar field. Even with 767 bytes the index size grows very fast and for a large table (where it is most useful) most probably won't fit into memory.
From the other side, the only frequent case at least in my experience where I needed to index a long varchar field was a unique constraint. And in all those cases a composite index of some group id and MD5 from the varchar field was sufficient. The only problem is to mimick the case-insensitive collation (which considers accented charactes and not-accented equal), though in all my cases I anyway used binary collation, so it was not a problem.
UPD. Another frequent case for indexing a long varchar is ordering. For this case I usually define a separate indexed sorter field which is a prefix of 5-15 characters depending on data distribution. For me, a compact index is more preferable than rarely inaccurate ordering.
I'm trying to understand mysql data types, but i don't get the difference between the (Var-)BINARY data fields and the BLOB-Fields. What ist the difference between these types?
BLOB's can be as big as you want.
Also, reading the MySQL manual online:
BLOB and TEXT differ from VARBINARY and VARCHAR in the following ways:
There is no trailing-space removal for BLOB and TEXT columns when
values are stored or retrieved. Before MySQL 5.0.3, this differs from
VARBINARY and VARCHAR, for which trailing spaces are removed when
values are stored.
On comparisons, TEXT is space extended to fit the compared object,
exactly like CHAR and VARCHAR.
For indexes on BLOB and TEXT columns, you must specify an index prefix
length. For CHAR and VARCHAR, a prefix length is optional. See Section
7.5.1, “Column Indexes”.
BLOB and TEXT columns cannot have DEFAULT values.
The binary and varbinary types are binary strings whose actual values are stored in the table. The actual values blob (and text) types are stored elsewhere in the database with a 256 byte alias to that slot being placed in the table; the blob can therefore be "any" size (up to the max).
Searched the docs, but to no avail.
What is the maximum length for an enumeration value, i.e. the string literal?
The limit isn't on the length of the literal string, but rather on the table definition.
The MySQL documentation states that
Each table has an .frm file that contains the table definition. The server uses the following expression to check some of the table information stored in the file against upper limit of 64KB.
which is then followed by an ad-hoc equation expressing the approximate size of a table definition.
For a simple test, in a table with a couple fields already, I got my enum up to 63136 characters long, and the .frm was 71775 bytes big (slightly larger than 70KB), so the limit is approximate. At that point, MySQL complained #1117 - Too many columns, which is misleading to say the least.
Interestingly/oddly/worthwhile to note, the character set of the enum will change the maximum length. -- even if you're using normal characters which should only require 1 byte each.
An ENUM column can have a maximum of 65,535 distinct elements. (The practical limit is less than 3000.) A table can have no more than 255 unique element list definitions among its ENUM and SET columns considered as a group.
i have an innodb mysql table with a varchar(100) field called 'word'.
i created a btree index on an that field.
if i do a query like select * from table where word = 'non-linear', i get all variations of that word. so results would include (Non-linear, Non-Linear, non-Linear, etc...).
it seems to me that that index doesnt care about the capitalization.
does this mean that the index has 1 record for this word?
if so, is there a way to get a list of all the keys in the index so that i would essentially have a list of unique terms?
String comparisons are not case-sensitive for non-binary data types like varchar, char, text. For binary strings (BINARY, VARBINARY, BLOB), comparisons use the numeric values of the bytes in the operands; this means that for alphabetic characters, comparisons will be case sensitive.
More information available at Mysql Docs.