Lecture 8 Objectives Physical Database Design

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E246WFC G123RMR G889VDU H203PBR H311MHG … Primary Index Example cont. 3000 records, 13 bytes each 39 blocks, 78 records each Primary Index File … 7 bytes 6 bytes to read block Block access = ⎡log2 39⎤+ 1 = 6 (compare with 12 for a binary search) 30,000 records, 100 bytes each 3000 blocks, 10 records Main Data File E246WFC 7648378 F651DEK 3096275 G123RMR 7493874 G551JBA 2098377 G889VDU 6587969 G994PBR 5675789 H203PBR 4654786 H266MHU 6345234 H311MHG 5675489 H626RPG 5673455 Clustering Index Example Consider the same relation: Cars(reg#, engine#, make, model, price) Assume clustering field is: make Clustering Clustering Index File Clustering Block Main Data File Filed G551JBA 7648378 Fiat F651DEK 3096275 Ford 1 Filed Value Pointer Fiat Ford Renault Rover Vauxhall G123RMR 7493874 Ford E246WFC 2098377 Ford G889VDU 6587969 Renault H266MHU 5675789 Renault H203PBR 4654786 Renault G994PBR 6345234 Rover H311MHG 5675489 Rover H626RPG 5673455 Vauxhall 1 2 3 4 5 2 3 4 5 1024 block size 37 39 Clustering Index • If records of a file are physically ordered on a non-key field. That field is called the clustering field. • A clustering index differs from a primary index, which requires that the ordering field of the data file have a distinct value for each record. • There is one entry in the clustering index for each distinct value of the clustering field. This is an example of a dense index. • A data file can have at most one primary index or one clustering index. Secondary Index • A secondary index is also an ordered file with two fields. The first field is of the same type as some non-ordering field of the data file. The second field is a block pointer or a record pointer. • There can be many secondary indexes for the same data file. 38 40
Example 1: Dense Secondary Index Say we had the same relation: Cars(reg#, engine#, make, model, price) Assume that engine# (secondary key) is 7 bytes long. A secondary key field has a distinct value for each data record. Hence, the secondary index will be dense. We want to know if it would be better to use a secondary index file constructed with engine# as the secondary index key, or Perform a simple linear search on the data file (cost = 1500 accesses). Example 2: Non-dense Secondary Index • We can also create a secondary index on a non-key field of a file. In this case, many records in the data file can have the same value for the indexing field. There are several options for implementing such an index. • The option which is more commonly used is to have a single entry for each index filed value, but to create an extra level of indirection. In this non-dense scheme, the secondary index pointer will point to a block of record pointers; each record pointer points to one record in the data file. 41 43 Example 1: Dense Secondary Index 30000 records, 13 bytes each 385 blocks, 78 records each Secondary Index File 2098377 3096275 4654786 5673455 5675489 5675789 6345234 6587969 7493874 7648378 7 bytes 6 bytes 30,000 records, 100 bytes each 3000 blocks, 10 records each Main Data File E246WFC 7648378 F651DEK 3096275 G123RMR 7493874 G551JBA 2098377 G889VDU 6587969 G994PBR 5675789 H203PBR 4654786 H266MHU 6345234 H311MHG 5675489 H626RPG 5673455 to read block Block access = ⎡log2 385⎤ + 1 = 10 (Better than a linear search on the data file which needs 1500 accesses) Example 2: Non-dense Secondary Index 30 records, 26 bytes each 1 block, 30 records in it Secondary Index File Fiat Ford Renault Rover Vauxhall 20 bytes 6 bytes Pointers to read block and pointer 30,000 records, 100 bytes each 3000 blocks, 10 records Main Data File E246WFC 7648378 Fiat F651DEK 3096275 Rover G123RMR 7493874 Ford G551JBA 2098377 Rover G889VDU 6587969 Renault G994PBR 5675789 Vauxhall H203PBR 4654786 Rover H266MHU 6345234 Ford H311MHG 5675489 Renault H626RPG 5673455 Fiat Block access = ⎡log2 1⎤ + 1+ 2 = 3 (Better than a linear search on the data file which needs 1500 accesses) 1 2 3 4 5 1024 block size 42 1 2 3 4 5 44 1024 block size
Page 1 and 2: Lecture 8 Physical Database Design
Page 3 and 4: PropertyforRent Relation and Staff
Page 5 and 6: Cross-Referencing Transactions and
Page 7 and 8: Ordered Files Find Say we wanted to
Page 9: Step 2.3 Choose Indexes To determin

Lecture 8 Objectives Physical Database Design

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