Query processing involves translating high-level queries into executable low-level queries, with key steps including parsing, optimization, and evaluation. The cost of queries is primarily measured through disk access, CPU time, and network communication, with disk access being crucial for large databases due to slower speeds compared to CPU. Cost calculation for disk access includes the number of seeks, blocks read, and blocks written, impacting overall query efficiency.

1. Query Processing:
Measuring Query
Cost

2. Query Processing
Query processing is the process of translating a
high-level query, such as SQL, into a low-level
query that can be executed by the database
system.
The steps involved are:
1. Parsing and translation.
2. Optimization.
3. Evaluation.

3. Steps in Query Processing

4. Query Cost
Query cost is the time taken by the query to hit
the database and return the result.
It involves query processing time i.e. time taken
to parse and translate the query, optimize it,
evaluate, execute and return the result to the
user

5. Factors in Measuring Query Cost
1
Disk access
The cost to access data from disk is
most important for large database
since the disk access are slow
compare to main memory access
2
CPU time
Time to execute the query code by
CPU
3
Communication time in Network
Time taken to the amount of data
transferred over the network during
a query execution

6. Measuring Query Cost
Why we prefer disk access for measure of query cost?
●
CPU speed > disk speed for executing a query. So it is difficult to
calculate with CPU time
Primarily considered with Distributed/ parallel system, it is difficult
to calculate transmit time in network
●

7. Query Cost: Disk access
No. of Seeks(N);
No. of blocks read;
No. of blocks write;
Cost = N x Avg Seek time
Cost = N x Avg block read cost
Cost = N x Avg block write cost
=> Disk Access cost is no of blocks transfer from disk and no of seeks
tT = time to transfer one block
tS = time for one to seek
Cost for b block transfers plus S seeks
= b * tT + S * tS
Best case: When all records resides in main memory
Worst case: When record resides in secondary memory or one part of record
resides in main memory and other in secondary memory

8. Thank you