Mysql what kind of table

Prefix compression is used on keys that start with a string. Prefix compression helps if there are many strings with an identical prefix. This helps when you have many integer keys that have an identical prefix when the numbers are stored high byte first. MyISAM supports 3 different table types. Two of them are chosen automatically depending on the type of columns you are using.

The third, compressed tables, can only be created with the myisampack tool. This is the default format. This is the simplest and most secure format. It is also the fastest of the on-disk formats. The speed comes from the easy way data can be found on disk.

When looking up something with an index and static format it is very simple. Just multiply the row number by the row length. Also, when scanning a table it is very easy to read a constant number of records with each disk read.

The security is evidenced if your computer crashes when writing to a fixed-size MyISAM file, in which case myisamchk can easily figure out where each row starts and ends.

So it can usually reclaim all records except the partially written one. Note that in MySQL all indexes can always be reconstructed:. This format is a little more complex because each row has to have a header that says how long it is. One record can also end up at more than one location when it is made longer at an update. Each record is preceded by a bitmap indicating which columns are empty " for string columns, or zero for numeric columns.

If a string column has a length of zero after removal of trailing spaces, or a numeric column has a value of zero, it is marked in the bitmap and not saved to disk. Non-empty strings are saved as a length byte plus the string contents.

Each record uses only as much space as is required. If a record becomes larger, it is split into as many pieces as are required. This results in record fragmentation. If you update a row with information that extends the row length, the row will be fragmented.

In this case, you may have to run myisamchk -r from time to time to get better performance. Not as easy to reconstruct after a crash because a record may be fragmented into many pieces and a link fragment may be missing.

There is a penalty of 6 bytes for each link. A dynamic record is linked whenever an update causes an enlargement of the record. Each new link will be at least 20 bytes, so the next enlargement will probably go in the same link. If not, there will be another link. Then, whenever you need age, you can calculate it as the difference between the current date and the birth date. MySQL provides functions for doing date arithmetic, so this is not difficult.

Storing birth date rather than age has other advantages, too:. You can use the database for tasks such as generating reminders for upcoming pet birthdays.

If you think this type of query is somewhat silly, note that it is the same question you might ask in the context of a business database to identify clients to whom you need to send out birthday greetings in the current week or month, for that computer-assisted personal touch.

You can calculate age in relation to dates other than the current date. For example, if you store death date in the database, you can easily calculate how old a pet was when it died. You can probably think of other types of information that would be useful in the pet table, but the ones identified so far are sufficient: name, owner, species, sex, birth, and death.

The options are reserved for future use. Permitted values are a string literal containing a valid JSON document or an empty string ''. Invalid JSON is rejected. In this case, the last specified value is used. Used to specify a generated column expression. For information about generated columns , see Section Stored generated columns can be indexed.

InnoDB supports secondary indexes on virtual generated columns. For general background in addition to the following descriptions, see Section The symbol value, if used, must be unique per schema database , per constraint type.

A duplicate symbol results in an error. See also the discussion about length limits of generated constraint identifiers at Section 9. The SQL standard specifies that all types of constraints primary key, unique index, foreign key, check belong to the same namespace.

In MySQL, each constraint type has its own namespace per schema. Consequently, names for each type of constraint must be unique per schema, but constraints of different types can have the same name. Each secondary index entry contains a copy of the primary key columns for the corresponding row. Doing so only marks that single column as primary. This was implemented for compatibility with other database systems.

An error occurs if you try to add a new row with a key value that matches an existing row. If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within the prefix length. Indexing always happens over the entire column; column prefix indexing is not supported and any prefix length is ignored if specified.

MySQL supports foreign keys, which let you cross-reference related data across tables, and foreign key constraints, which help keep this spread-out data consistent. Partitioned tables employing the InnoDB storage engine do not support foreign keys. The default is ascending if no order specifier is given. Prefix limits are measured in bytes. Take this into account when specifying a prefix length for a nonbinary string column that uses a multibyte character set.

Beginning with MySQL 8. Multi-Valued Indexes , provides detailed information regarding creation of, usage of, and restrictions and limitations on multi-valued indexes. Some storage engines permit you to specify an index type when creating an index. It can be given before the column list, but support for use of the option in that position is deprecated and you should expect it to be removed in a future MySQL release.

The value is treated as a hint; a different size could be used if necessary. It associates a parser plugin with the index if full-text indexing and searching operations need special handling. Index definitions can include an optional comment of up to characters. Specify index visibility. Indexes are visible by default. An invisible index is not used by the optimizer. Specification of index visibility applies to indexes other than primary keys either explicit or implicit.

For more information, see Section 8. For more information about indexes, see Section 8. The columns of the referenced table must always be explicitly named.

For more detailed information and examples, see Section In that case, the child table row containing such a foreign key is permitted to be inserted, and does not match any row in the referenced parent table. It is possible to implement other semantics using triggers. Additionally, MySQL requires that the referenced columns be indexed for performance.

For more information, see Section 1. Table options are used to optimize the behavior of the table. In most cases, you do not have to specify any of them. These options apply to all storage engines unless otherwise indicated. Options that do not apply to a given storage engine may be accepted and remembered as part of the table definition. Specifies the storage engine for the table, using one of the names shown in the following table.

The engine name can be unquoted or quoted. By default, if a storage engine is specified that is not available, the statement fails with an error. Defines the amount by which InnoDB extends the size of the tablespace when it becomes full.

Introduced in MySQL 8. The setting must be a multiple of 4MB. The default setting is 0, which causes the tablespace to be extended according to the implicit default behavior.

The value cannot be set lower than the maximum value currently in the column. An approximation of the average row length for your table. You need to set this only for large tables with variable-size rows. If your operating system does not support files that large, table sizes are constrained by the file size limit.

They are also optimal for performance. InnoDB table supports foreign keys , commit, rollback, roll-forward operations. The underlying tables will not be affected. The memory tables are stored in memory and use hash indexes so that they are faster than MyISAM tables.

The lifetime of the data of the memory tables depends on the uptime of the database server.