How to store null value on the disk of yyds dry inventory?

1 Why not save it directly NULL？

NULL List of values , The possible field values in a row of data are NULL, such as nickname Field , Allow for NULL, When the storage , If there is no assignment , The field value is NULL.
Suppose this field is NULL When the value is stored on disk , Is in accordance with the “NULL” String storage , Isn't it a waste of storage space and strange ？

2 How to store ？

Not through string , But through binary bit Bit storage , It is assumed that the values of multiple fields in a row of data are NULL, So the of these fields NULL, It'll take bit The bit form is stored in NULL List of values .
The following table ：

Log in and copy

      
      

       
       CREATE 
       
       TABLE customer 
       
       (
       
       
  name 
       
       VARCHAR
       
       (
       
       10
       
       ) 
       
       NOT 
       
       NULL
       
       ,
       
       
  address 
       
       VARCHAR
       
       (
       
       20
       
       )
       
       , gender 
       
       CHAR
       
       (
       
       1
       
       )
       
       ,
       
       
  job 
       
       VARCHAR
       
       (
       
       30
       
       )
       
       ,
       
       
  school 
       
       VARCHAR
       
       (
       
       50
       
       )
       
       

       
       ) ROW_FORMAT
       
       =COMPACT
       
       ;
      
      
      
      

       
       
1.
       
       
2.
       
       
3.
       
       
4.
       
       
5.
       
       
6.
      
      

Yes 4 Variable length fields , There is also a fixed length field ,name Statement NOT NULL, other 4 All fields are possible NULL
How can the following row of data be stored on disk ：

Log in and copy

      
      

       
       jack NULL m NULL xx_school
      
      
      
      

       
       
1.
      
      

There are two fields NULL

3 Disk storage format of a row of data

Think about the row of case data in the table above , How to store on disk , Because it has multiple variable length fields , Multiple fields are allowed to be NULL. First of all, let's review , The storage format of a row of data on disk should be as follows ：

Log in and copy

      
      

       
        Variable length field length list  
       
       NULL List of values   Header information  column1
       
       =value1 column2
       
       =value2 ... columnN
       
       =valueN
      
      
      
      

       
       
1.
      
      

4 Variable length fields , Put first in reverse order school Length of field , Put again job、address、name The value length of several fields ？
But there is one problem to distinguish , If this variable length field value is NULL, There is no need to store its value length in the variable length field length list , So the data in the above line , Only name and school Two variable length fields have values , Put their lengths in the variable length field length list in reverse order ：

Log in and copy

      
      

       
       0x09 
       
       0x04 
       
       NULL List of values   Header information  column1
       
       =value1 column2
       
       =value2 ... columnN
       
       =valueN
      
      
      
      

       
       
1.
      
      

All allowable values are NULL Field of , It doesn't have to be worth NULL, As long as it is allowed to NULL Field of , Each field has a binary bit A value ：

• bit The value is 1 That is the NULL
• bit The value is 0 The explanation is not NULL

For example, above 4 Both fields are allowed to be NULL, Everyone will have one bit position , The value of this row of data is

Log in and copy

      
      

       
       jack NULL m NULL xx_school
      
      
      
      

       
       
1.
      
      

among 2 The fields are null,2 A field is not null, therefore 4 individual bit It should be ：1010
But in fact NULL Value list , Put in reverse order , therefore NULL In the value list ：0101, The overall data in this row is as follows ：

Log in and copy

      
      

       
       0x09 0x04 0101  Header information  column1=value1 column2=value2 ... columnN=valueN
      
      
      
      

       
       
1.
      
      

actual NULL When storing the value list , Generally at least 8 individual bit Multiple of bits , Insufficient 8 individual bit The high position is filled 0, So the actual storage is as follows ：

Log in and copy

      
      

       
       0x09 0x04 00000101  Header information  column1=value1 column2=value2 ... columnN=valueN
      
      
      
      

       
       
1.
      
      

4 How to read a row of data on the disk ？

Disk data storage format ：

Log in and copy

      
      

       
       0x09 0x04 00000101  Header information  column1=value1 column2=value2 ... columnN=valueN
      
      
      
      

       
       
1.
      
      

First reading ：

• Variable length field length list

You know there are several variable length fields

• NULL List of values

Which variable length fields are NULL
From the variable length field length list, it is resolved that it is not NULL Value length of variable length field , Then you also know which fields are NULL. Based on this information , From the actual column value storage area , Read out the value of each field .

• Variable length field value , Just read by the length of the value , if NULL, I knew he was a NULL, No value stored
• Fixed length field , Read according to the fixed length