File Processing : Query Processing Spatiotemporal Database Laboratory Pusan National University 2004, Spring Pusan National University Ki-Joune Li

Basic Concepts of Query Retrieve records satisfying predicates Types of Query Operators Aggregate Query Sorting Spatiotemporal Database Laboratory Pusan National University

Relational Operators : Select Selection (condition) Retrieve records satisfying predicates Example Find Student where Student.Score > 3.5 score>3.5(Student) Index or Hash Predicate Spatiotemporal Database Laboratory Pusan National University Select

Relational Operators : Project Project (attributes) Extract interesting attributes Example Find Student.name where score > 3.5 name(acore>3.5(Student)) Full Scan Interesting attributes to get Spatiotemporal Database Laboratory Pusan National University Extract

Cartisan Product Cartisan Product () Join ( ) Two Tables : R1  R2 Produce all cross products Join ( ) r11 … r21 r22 r2n r12 r1m r11 r12 … r1m R1 r21 r22 … r2n R2 Spatiotemporal Database Laboratory Pusan National University  =

Join Join ( ) Select combined records of cartisan product with same value of a common attribute (Natural Join) Example Student (StudentName, AdvisorProfessorID, Department, Score) Professor(ProfessorName, ProfessorID, Department) Student AdivsorProfessorID=ProfessorID Professor =  AdivsorProfessorID=ProfessorID(Student Professor) Double Scan : Expensive Operation Spatiotemporal Database Laboratory Pusan National University

Relational Algebra Relational Algebra Operand : Table (Relation) Operator : Relational Operator (, , , etc) Example Find Student Name where Student Score > 3.5 and Advisor Professor belongs to CSE Department student.name(acore>3.5(Student) Department=‘CSE’ (Professor) ) Relational Algebra Specifies the sequence of operations Spatiotemporal Database Laboratory Pusan National University   

Query Processing Mechanism Query Processing Steps 1. Parsing and translation 2. Optimization 3. Evaluation Spatiotemporal Database Laboratory Pusan National University

Parsing and Translation Parsing Query Statement (e.g. in SQL) Translation into relational algebra Equivalent Expression For a same query statement several relation algebraic expressions are possible Example balance  2500(name(account ))  name(balance  2500(account )) Different execution schedules Query Execution Plan (QEP) Determined by relational algebra Several QEPs may be produced by Parsing and Translation Spatiotemporal Database Laboratory Pusan National University

Query Optimization Choose ONE QEP among QEPs with the consideration on Execution Cost of each QEP Cost : Execution Time How to find cost of QEP ? Real Execution Exact but Not Feasible Cost Estimation Types of Operations Number of Records Selectivity Distribution of data Spatiotemporal Database Laboratory Pusan National University

Cost Model : Basic Concepts Cost Model : Number of Block Accesses Cost C = Cindex + Cdata where Cindex : Cost for Index Access Cdata : Cost for Data Block Retrieval Cindex vs. Cdata ? Cindex : depends on index Cdata depends on selectivity Random Access or Sequential Access Selectivity Number (or Ratio) of Objects Selected by Query Spatiotemporal Database Laboratory Pusan National University

Cost Model : Type of Operations Cost model for each type of operations Select Project Join Aggregate Query Query Processing Method for each type of operations Index/Hash or Not Spatiotemporal Database Laboratory Pusan National University

Cost Model : Number of Records Nrecord  Nblocks Number of Scans Single Scan O(N) : Linear Scan O(logN ) : Index Multiple Scans O(NM ) : Multiple Linear Scans O(N logM ) : Multiple Scans with Index Spatiotemporal Database Laboratory Pusan National University

Selectivity Selectivity Affects on Cdata Random Access Scattered on several blocks Nblock  Nselected Sequential Access Contiguously stored on blocks Nblock = Nselected / Bf Spatiotemporal Database Laboratory Pusan National University

Selectivity Estimation Depends on Data Distribution Example Q1 : Find students where 60 < weight < 70 Q2 : Find students where 80 < weight < 90 How to find the distribution Parametric Method e.g. Gaussian Distribution No a priori knowledge Non-Parametric Method e.g. Histogram Smoothing is necessary Wavelet, Discrete Cosine Spatiotemporal Database Laboratory Pusan National University 30 40 50 60 70 80 90 100 Frequency

Select : Linear Search Algorithm : linear search Scan each file block and test all records to see whether they satisfy the selection condition. Cost estimate (number of disk blocks scanned) = br br denotes number of blocks containing records from relation r If selection is on a key attribute (sorted), cost = (br /2) stop on finding record Linear search can be applied regardless of selection condition or ordering of records in the file, or availability of indices Spatiotemporal Database Laboratory Pusan National University

Select : Range Search Algorithm : primary index, comparison Relation is sorted on A For A  V (r) Step 1: use index to find first tuple  v and Step 2: scan relation sequentially For AV (r) just scan relation sequentially till first tuple > v; do not use index Algorithm : secondary index, comparison Step 1: use index to find first index entry  v and Step 2: scan index sequentially to find pointers to records. scan leaf nodes of index finding pointers to records, till 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 if records are scattered on many blocks Spatiotemporal Database Laboratory Pusan National University

Select : Complex Query Conjunction : 1  2 . . . n(r) Algorithm : selection using one index Step 1: Select a combination of i (i (r) ) Step 2: Test other conditions on tuple after fetching it into memory buffer. Algorithm : selection using multiple-key index Use appropriate multiple-attribute index if available. Algorithm : selection by intersection of identifiers Step 1: Requires indices with record pointers. Step 2: Intersection of all the obtained sets of record pointers. Step 3: Then fetch records from file Disjunction : 1 2 . . . n (r) Algorithm : Disjunctive selection by union of identifiers Spatiotemporal Database Laboratory Pusan National University

Join Operation Several different algorithms to implement joins Nested-loop join Block nested-loop join Indexed nested-loop join Merge-join Hash-join Choice based on cost estimate Examples use the following information Number of records of customer: 10,000 depositor: 5000 Number of blocks of customer: 400 depositor: 100 Spatiotemporal Database Laboratory Pusan National University

Nested-Loop Join Algorithm NLJ the theta join r  s For each tuple tr in r do begin For each tuple ts in s do begin test pair (tr,ts) to see if they satisfy the join condition  if they do, add tr • ts to the result. End End r : outer relation, s : inner relation. No indices, any kind of join condition. Expensive Spatiotemporal Database Laboratory Pusan National University

Nested-Loop Join (Cont.) In the worst case, if there is enough memory only to hold one block of each relation, the estimated cost is nr  bs + br disk accesses. If the smaller relation fits entirely in memory, use that as the inner relation. Reduces cost to br + bs disk accesses. Assuming worst case memory availability cost estimate is 5000  400 + 100 = 2,000,100 disk accesses with depositor as outer relation, and 1000  100 + 400 = 1,000,400 disk accesses with customer as the outer relation. If smaller relation (depositor) fits entirely in memory, the cost estimate will be 500 disk accesses. Block nested-loops algorithm (next slide) is preferable. Spatiotemporal Database Laboratory Pusan National University

Block Nested-Loop Join Algoritm BNLJ For each block Br of r do begin For each block Bs of s do begin For each tuple tr in Br do begin For each tuple ts in Bs do begin Check if (tr,ts) satisfy the join condition if they do, add tr • ts to the result. End End End End Spatiotemporal Database Laboratory Pusan National University

Block Nested-Loop Join (Cont.) Worst case estimate: br  bs + br block accesses. Each block in the inner relation s is read once for each block in the outer relation (instead of once for each tuple in the outer relation Best case: br + bs block accesses. Improvements to nested loop and block nested loop algorithms: In block nested-loop, use (M-2) disk blocks as blocking unit for outer relations, where M = memory size in blocks; use remaining two blocks to buffer inner relation and output Cost = br / (M-2)  bs + br If equi-join attribute forms a key or inner relation, stop inner loop on first match Scan inner loop forward and backward alternately, to make use of the blocks remaining in buffer (with LRU replacement) Use index on inner relation if available (next slide) Spatiotemporal Database Laboratory Pusan National University

Indexed Nested-Loop Join Index lookups can replace file scans if join is an equi-join or natural join and an index is available on the inner relation’s join attribute Can construct an index just to compute a join. For each tuple tr in the outer relation r, use the index to look up tuples in s that satisfy the join condition with tuple tr. Worst case: buffer has space for only one page of r, and, for each tuple in r, we perform an index lookup on s. Cost of the join: br + nr  c Where c is the cost of traversing index and fetching all matching s tuples for one tuple or r c can be estimated as cost of a single selection on s using the join condition. If indices are available on join attributes of both r and s, use the relation with fewer tuples as the outer relation. Spatiotemporal Database Laboratory Pusan National University

Example of Nested-Loop Join Costs Compute depositor customer, with depositor as the outer relation. Let customer have a primary B+-tree index on the join attribute customer-name, which contains 20 entries in each index node. Since customer has 10,000 tuples, the height of the tree is 4, and one more access is needed to find the actual data depositor has 5000 tuples Cost of block nested loops join 400*100 + 100 = 40,100 disk accesses assuming worst case memory (may be significantly less with more memory) Cost of indexed nested loops join 100 + 5000 * 5 = 25,100 disk accesses. CPU cost likely to be less than that for block nested loops join Spatiotemporal Database Laboratory Pusan National University

Hash-Join Applicable for equi-joins and natural joins. A hash function h is used to partition tuples of both relations h maps JoinAttrs values to {0, 1, ..., n}, where JoinAttrs denotes the common attributes of r and s used in the natural join. r0, r1, . . ., rn denote partitions of r tuples Each tuple tr  r is put in partition ri where i = h(tr [JoinAttrs]). s0, s1. . ., sn denotes partitions of s tuples Each tuple ts s is put in partition si, where i = h(ts [JoinAttrs]). Note: In book, ri is denoted as Hri, si is denoted as Hsi and n is denoted as nh. Spatiotemporal Database Laboratory Pusan National University

Hash-Join (Cont.) Spatiotemporal Database Laboratory Pusan National University

Hash-Join (Cont.) r tuples in ri need only to be compared with s tuples in si . Need not be compared with s tuples in any other partition, since: an r tuple and an s tuple that satisfy the join condition will have the same value for the join attributes. If that value is hashed to some value i, the r tuple has to be in ri and the s tuple in si . Spatiotemporal Database Laboratory Pusan National University

The hash-join of r and s is computed as follows. Hash-Join Algorithm The hash-join of r and s is computed as follows. 1. Partition the relation s using hashing function h. When partitioning a relation, one block of memory is reserved as the output buffer for each partition. 2. Partition r similarly. 3. For each i: (a) Load si into memory and build an in-memory hash index on it using the join attribute. This hash index uses a different hash function than the earlier one h. (b) Read the tuples in ri from the disk one by one. For each tuple tr locate each matching tuple ts in si using the in-memory hash index. Output the concatenation of their attributes. Spatiotemporal Database Laboratory Pusan National University Relation s is called the build input and r is called the probe input.