Chapter 12: Query Processing
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1 Chapter 12: Query Processing Database System Concepts, 6 th Ed. See for conditions on re-use
2 Basic Steps in Query Processing 1. Parsing and translation 2. Optimization 3. Evaluation 12.2
3 Basic Steps in Query Processing Parsing and translation translate the query into its internal form. This is then translated into relational algebra. Parser checks syntax, verifies relations Evaluation The query-execution engine takes a query-evaluation plan, executes that plan, and returns the answers to the query. 12.3
4 Basic Steps in Query Processing : Optimization A relational algebra expression may have many equivalent expressions E.g., salary ( salary (instructor)) is equivalent to salary ( salary (instructor)) Each relational algebra operation can be evaluated using one of several different algorithms Correspondingly, a relational-algebra expression can be evaluated in many ways. 12.4
5 Optimization An annotated relational algebra expression specifying the detailed evaluation strategy is called an evaluation primitive Example: We can use an index on salary to find instructors with salary < 75000, or we can perform a complete (serial) relation scan and discard instructors with salary The sequence of annotated relational algebra expressions is called an evaluation plan. 12.5
6 Basic Steps: Optimization The goal of Query Optimization: Amongst all equivalent evaluation plans choose the one with the lowest cost. How do we measure cost? Number of tuples in each relation Size of tuples Number of disk access operations CPU time RAM space needed Time needed for communications over a network (LAN, SAN) Etc. 12.6
7 QUIZ: Query Processing What are the 3 main steps in query processing? What is an evaluation primitive? What is a query evaluation plan? 12.7
8 QUIZ: Query Processing What are the 3 main steps in query processing? What is an evaluation primitive? What is a query evaluation plan? 12.8
9 12.2 Measures of Query Cost Time cost = total elapsed time for answering the query Many factors contribute to time cost Typically disk access is the predominant time cost (also relatively easy to estimate!). Measured by taking into account Number of seeks Number of blocks read * average-seek-cost * average-block-read-cost Number of blocks written * average-block-write-cost Cost to write a block is greater than cost to read a block b/c data is read back after being written to ensure that the write was free of errors! 12.9
10 12.2 Measures of Query Cost Cost to write a block is greater than cost to read a block b/c data is read back after being written to ensure that the write was free of errors! 12.10
11 Measures of Query Cost Simplification: we just use the number of block transfers from disk and the number of seeks as the cost measures t T time to transfer one block t S time for one seek Cost for b block transfers plus S seeks b * t T + S * t S Simplification: we ignore CPU costs (time) 0.1 ms Real systems do take CPU cost into account 4 ms Simplification: we do not include cost to writing the output to disk. Why? All execution plans we must choose from end up with the same data 12.11
12 QUIZ: Measures of Query Cost 0.1 ms 4 ms Cost for b block transfers plus S seeks b * t T + S * t S If the size of a block is 4 KB, and the transfer rate is 200 MB/s, calculate the block transfer time
13 QUIZ: Measures of Query Cost 0.1 ms Cost for b block transfers plus S seeks b * t T + S * t S 4 ms If the size of a block is 4 KB, and the transfer rate is 200 MB/s, calculate the block transfer time. 4 KB / 200 MB/s = 4/(200*1024) s = s = = ms 0.02 ms Note: Today s best enterprise level HDDs have transfer rates around 200 Mb/s when handling pairs of consecutive blocks (2x4 KB = 8 KB), see for example this test on a Seagate drive
14 QUIZ: Measures of Query Cost Cost for b block transfers plus S seeks b * t T + S * t S Evaluation of a query requires 3 disk seeks, each followed by transfer of 50 blocks. What is the total time cost? 0.1 ms 4 ms 12.14
15 QUIZ: Measures of Query Cost Cost for b block transfers plus S seeks b * t T + S * t S Evaluation of a query requires 3 disk seeks, each followed by transfer of 50 blocks. What is the total time cost? 0.1 ms 4 ms 3*50* 0.1 ms + 3*4 ms = 15 ms + 12 ms = 27 ms 3*50*0.02 ms + 3*4 ms = 3 ms + 12 ms = 15 ms If using transfer time from prev. example 12.15
16 Perspective: Transfer Rates Theoretical maximum for SATA: 600 MB/s (SATA III), but in practice ~ MB/s SATA Express announced in 2014: 2 GB/s (theoretical) PCI Express (PCIe) 3.0 bus: ~1 GB/s per lane, with typical 16x configurations : SSD connected through PCI have surpassed 1 GB/s SATA = Serial ATA (Advanced Technology Attachment) PCI = Periferal Component Interconnect
17 12.3 Selection Operation Selection w/o an index is a file scan Algorithm A1 (linear search). Scan each file block and test all records to see whether they satisfy the selection condition. Cost estimate: 1 seek + b r block transfers b r denotes number of blocks containing records from relation r If selection is on a key attribute, can stop on finding record cost = 1 seek + (b r /2) block transfers Linear search can be applied regardless of selection condition or ordering of records in the file, or availability of indices average 12.17
18 Extra-credit 12.18
19 QUIZ: Linear search A table has 50,000 tuples, each 100 Bytes in length. What is the average time cost using linear search? State all your assumptions! 12.19
20 QUIZ: Linear search A table has 50,000 tuples, each 100 Bytes in length. What is the average time cost using linear search? State all your assumptions! The table has a total of 50,000*100 = 5 million B 5 million B / 4096 B/block = 1,220.7 blocks 1,220.7 / 2 = blocks on average 611 * 0.1 ms + 4 ms = 65.1 ms 12.20
21 Why not binary search? 12.3 Selection Operation Generally does not make sense since data is not stored consecutively except when there is an index available, and binary search requires more seeks than index search 12.21
22 Selections Using Indices Index scan search algorithms that use an index selection condition must be on search-key of index! A2 (primary index, equality on key). Retrieve a single record that satisfies the corresponding equality condition Cost = (h i + 1) * (t T + t S ) A3 (primary index, equality on nonkey) Retrieve multiple records (duplicates). Records will be in consecutive blocks (Why?) Let b = number of blocks containing matching records Cost = h i * (t T + t S ) + t S + t T * b 12.22
23 Extra-credit 12.23
24 QUIZ: Linear search A table has 50,000 tuples, each 100 Bytes in length. The index is a B+ tree with n = 100. In our table, there are no more than 1000 duplicates for any nonkey value. What is the average time cost using primary index, equality on nonkey? State all your assumptions! 12.24
25 Selections Using Indices A4 (secondary index, equality on nonkey). Retrieve a single record if the search-key is a candidate key Cost = (h i + 1) * (t T + t S ) Retrieve multiple records if search-key is not a candidate key each of n matching records may be on a different block Cost = (h i + n) * (t T + t S ) Can be very expensive! 12.25
26 Selections Involving Comparisons Can implement selections of the form A V (r) or A V (r) by using a linear file scan, or by using indices in the following ways: A5 (primary index, comparison). (Relation is sorted on A) For A V (r) use index to find first tuple v and scan relation sequentially from there For A V (r) just scan relation sequentially until first tuple > v; do not use index A6 (secondary index, comparison). For A V (r) use index to find first index entry v and scan index sequentially from there, to find pointers to records. For A V (r) just scan leaf pages of index finding pointers to records, until first entry > v In either case, retrieve records that are pointed to requires an I/O for each record Linear file scan may be cheaper 12.26
27 This is the end of the material covered in our class We stop on p.545, before Complex Selections Review today in the lab! 12.27
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