Slideshow:
Structured and unstructured data
Structured data can have
fixed length and
variable length record fields
How records are stored inside a
disk block
Information stored in the
block header
Fill ordering of
records in a disk block
Offsets in the block header give the location in the disk block where a record is stored.
- Types of
data:
- Structured data:
- Structured data has
a common structure in
the data format
Example: all data titem has the following format
- The format is called
the record format
- A structured data item is called a record
- Structured data has
a common structure in
the data format
- Unstructured data:
- There is no common
structure in the
data items stored
Example: webpages on the World Wide Web.
- There is no common
structure in the
data items stored
Database Systems only manage structured data !!!
- Structured data:
- Types of
record fields:
- A record field can have
a fixed length
When all record fields are fixed length, then the record is a fixed length record:
- A record field can have
a variable length
When at least one record field is variable length, then the record is a variable length record:
- A record field can have
a fixed length
- We will
study
how to store
records in
a file on
disk:
- General practices on how to store records
- How to store records with only fixed length fields
- How to store records with some variable length fields
- File:
- Records are stored
in a file
- A file consists of one or more disk blocks
So what we really are studying is:
- How do we store records inside disk blocks
- Records are stored
in a file
- Fact:
- There is a block structure (organization) inside a disk block when records are stored in a (disk) block
Graphically:
- Structure
of a (disk) block:
-
Block header
(contains information on the
records stored in this
(disk) block)
- The records
themselves
-
Block header
(contains information on the
records stored in this
(disk) block)
- Block header:
- Information stored in a
block header:
- The role of the block (e.g., overflow block)
- The source relation of the record stored in the block
- The locations (offsets) of each record stored in the block
- Link(s) to other blocks (e.g., B+-tree - later)
- Information stored in a
block header:
- To maximize the
space utilization,
the positioning of
the block header information and
records are
as follows:
Notice:
- The block header contains
a table of offsets of the
records stored in the block
- The table of offsets
grows from
left to right (⇒)
The records in the block are insert from right to left (⇐)
- The block header contains
a table of offsets of the
records stored in the block