MySQL brochure notes 5 InnoDB record storage structure

MySQL Booklet notes 5 InnoDB Record storage structure

1. InnoDB yes MySQL The default storage engine

2. The speed of reading and writing to disk is very slow , And memory read and write a few orders of magnitude

Dividing data into pages , With page As an interface between disk and memory Basic unit ,InnoDB The size of the middle page is generally 16 KB.
That is to say, in general , Read at least from disk at a time 16KB In memory , Put at least... In memory at a time 16KB Refresh content to disk .

3. We usually use Record Unit to insert data into the table , The way these records are stored on disk is also called Line format perhaps Record format .

4 A common row format is Compact、Redundant、Dynamic and Compressed Line format .

4. Specify the row format

CREATE TABLE Table name ( Column information ) ROW_FORMAT= Line format name ;

ALTER TABLE Table name ROW_FORMAT= Line format name ;

CREATE TABLE record_format_demo (
    c1 VARCHAR(10),
    c2 VARCHAR(10) NOT NULL,
    c3 CHAR(10),
    c4 VARCHAR(10)
) CHARSET=ascii ROW_FORMAT=COMPACT;

INSERT INTO record_format_demo(c1, c2, c3, c4) VALUES('aaaa', 'bbb', 'cc', 'morningcat2018'), ('eeee', 'fff', NULL, NULL);

+------+-----+------+---------------------+
| c1   | c2  | c3   |          c4         |
+------+-----+------+---------------------+
| aaaa | bbb | cc   |   morningcat2018    |
| eeee | fff | NULL |         NULL        |
+------+-----+------+---------------------+

5. Compact Line format

• Additional information recorded
  • Variable length field length list
    • MySQL Support some variable length data types , such as VARCHAR(M)、VARBINARY(M)、 Various TEXT type , Various BLOB type ; Columns of these data types are called Variable length field
    • The storage space occupied by these variable length fields is divided into two parts ：1 Real data content ,2 Number of bytes occupied
    • The number of bytes occupied by the variable length field data is in the order of columns Storage in reverse order
      • In the first record c1、c2、c4 Columns are variable length fields , The lengths are 4,2,14 ; therefore Variable length field length list The value of is 0E0204 ( Hexadecimal representation )
      • In the second record c4 by NULL Don't record ; therefore Variable length field length list The value of is 0304
  • NULL List of values
    • Use 1bit Storage ,1 The value representing the column is NULL;0 The value representing the column is not NULL
    • Storage in reverse order
    • Zero up for high position
    • Only possible NULL Columns of are stored here ; therefore record_format_demo Tabular NULL List of values by 0000 0 c4 c3 c1
      • In the second record ,c3 c4 by NULL, The value is 1 ; therefore NULL List of values by 0000 0110 ; Hexadecimal is expressed as 06
  • Record header information
    • fixed 5 Bytes form (40 bit)
      • reserved 1 1bit
      • reserved 2 1bit
      • delete_mask 1bit Mark whether the record is deleted
      • min_rec_mask 1bit B+ This flag is added to the minimum record in each non leaf node of the tree
      • n_owned 4bit Indicates the number of records owned by the current record
      • heap_no 13bit Represents the location information of the current record in the record heap
      • record_type 3bit Indicates the type of current record ,0 It means ordinary record ,1 Express B+ Tree non leaf node records ,2 Represents the minimum record ,3 Indicates the maximum record
      • next_record 16bit Indicates the relative position of the next record
• Real data recorded
  • In addition to custom column data ,MySQL Some columns will be added by default for each record （ Also known as Hide columns ）
    • DB_ROW_ID Not necessary 6byte That's ok ID, Uniquely identify a record
    • DB_TRX_ID must 6byte Business ID
    • DB_ROLL_PTR must 7byte rollback pointer
  • InnoDB Table generation strategy for primary key
    • User defined primary key is preferred
    • No primary key defined , Then choose one Unique Key as primary key
    • even Unique If there's no definition of a key , be InnoDB A table named... Is added by default row_id As the primary key
  • The first 1 In records
    • c3 The value of the column , It is CHAR(10) Type of , The string it actually stores is ：‘cc’, and ascii The byte representation in the character set is ’0x6363’, Although it means that this string only takes 2 Bytes , But the whole c3 Columns still occupy 10 Bytes of space ,
    • Other than real data 8 All bytes are filled with space characters , The space character is in ascii The representation of a character set is 0x20

6. Redundant Line format

Redundant The line format is MySQL5.0 A line format used before

7. line overflow

A page is usually 16KB, When there is too much data in the record , When the current page doesn't fit , The extra data will be stored in other pages , This phenomenon is called line overflow