CS计算机代考程序代写 SQL data structure database >>

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Scanning

• Scanning
• Selection via Scanning
• Iterators
• Example Query
• next_tuple() Function
• Relation Copying
• Scanning in PostgreSQL
• Scanning in other File Structures
COMP9315 21T1 ♢ Scanning ♢ [0/16]
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❖ Scanning

Consider executing the query:

select * from Rel;
where the relation has a file structure like:



This would done by a simple scan of all records/tuples.
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❖ Scanning (cont)

Abstract view of how the scan might be implemented:

for each tuple T in relation Rel {
add tuple T to result set
}
Operational view:

for each page P in file of relation Rel {
for each tuple T in page P {
add tuple T to result set
}
}

Cost = read every data page once = b
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❖ Scanning (cont)

Consider a file with overflow pages, e.g.



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❖ Scanning (cont)

In this case, the implementation changes to:

for each page P in data file of relation Rel {
for each tuple t in page P {
add tuple t to result set
}
for each overflow page V of page P {
for each tuple t in page V {
add tuple t to result set
} } }
Cost: read each data page and each overflow page once
Cost = b + bOv
where bOv = total number of overflow pages
COMP9315 21T1 ♢ Scanning ♢ [4/16]
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❖ Selection via Scanning

Consider a one query like:

select * from Employee where id = 762288;
In an unordered file, search for matching tuple requires:






Guaranteed at most one answer; but could be in any page.
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❖ Selection via Scanning (cont)

Overview of scan process:

for each page P in relation Employee {
for each tuple t in page P {
if (t.id == 762288) return t
} }
Cost analysis for one searching in unordered file
• best case: read one page, find tuple
• worst case: read all b pages, find in last (or don’t find)
• average case: read half of the pages (b/2)
Page Costs:   Costavg = b/2    Costmin = 1    Costmax = b
COMP9315 21T1 ♢ Scanning ♢ [6/16]
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❖ Iterators

Access methods typically involve iterators, e.g.

Scan s = start_scan(Relation r, …)
• commence a scan of relation r
• Scan may include condition to implement WHERE-clause
• Scan holds data on progress through file (e.g. current page)

Tuple next_tuple(Scan s)
• return Tuple immediately following last accessed one
• returns NULL if no more Tuples left in the relation
COMP9315 21T1 ♢ Scanning ♢ [7/16]
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❖ Example Query

Example: simple scan of a table …

select name from Employee
implemented as:

DB db = openDatabase(“myDB”);
Relation r = openRelation(db,”Employee”,READ);
Scan s = start_scan(r);
Tuple t; // current tuple
while ((t = next_tuple(s)) != NULL) {
char *name = getStrField(t,2);
printf(“%s
”, name);
}

COMP9315 21T1 ♢ Scanning ♢ [8/16]
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❖ next_tuple() Function

Consider the following possible Scan data structure

typedef ScanData *Scan;

typedef struct {
Relation rel;
Page *curPage; // Page buffer
int curPID; // current pid
int curTID; // current tid
} ScanData;
Assume tuples are indexed 0..nTuples(p)-1
Assume pages are indexed 0..nPages(rel)-1
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❖ next_tuple() Function (cont)

Implementation of  Tuple next_tuple(Scan)  function

Tuple next_tuple(Scan s)
{
if (s->curTID >= nTuples(s->page)-1) {
// get a new page; exhausted current page
s->curPID++;
if (s->curPID >= nPages(s->rel))
return NULL;
else {
s->page = get_page(s->rel, s->curPID);
s->curTID = -1;
}
}
s->curTID++;
return get_tuple(s->rel, s->page, s->curTID);
}

COMP9315 21T1 ♢ Scanning ♢ [10/16]
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❖ Relation Copying

Consider an SQL statement like:

create table T as (select * from S);
Effectively, copies data from one table to a new table.
Process:

make empty relation T
s = start scan of S
while (t = next_tuple(s)) {
insert tuple t into relation T
}

COMP9315 21T1 ♢ Scanning ♢ [11/16]
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❖ Relation Copying (cont)

It is possible that T is smaller than S
• may be unused free space in S where tuples were removed
• if T is built by simple append, will be compact



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❖ Relation Copying (cont)

In terms of existing relation/page/tuple operations:

Relation in; // relation handle (incl. files)
Relation out; // relation handle (incl. files)
int ipid,opid,tid; // page and record indexes
Record rec; // current record (tuple)
Page ibuf,obuf; // input/output file buffers

in = openRelation(“S”, READ);
out = openRelation(“T”, NEW|WRITE);
clear(obuf); opid = 0;
for (ipid = 0; ipid < nPages(in); ipid++) { ibuf = get_page(in, ipid); for (tid = 0; tid < nTuples(ibuf); tid++) { rec = get_record(ibuf, tid); if (!hasSpace(obuf,rec)) { put_page(out, opid++, obuf); clear(obuf); } insert_record(obuf,rec); } } if (nTuples(obuf) > 0) put_page(out, opid, obuf);

COMP9315 21T1 ♢ Scanning ♢ [13/16]
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❖ Scanning in PostgreSQL

Scanning defined in: backend/access/heap/heapam.c
Implements iterator data/operations:
• HeapScanDesc … struct containing iteration state
• scan = heap_beginscan(rel,…,nkeys,keys)
• tup = heap_getnext(scan, direction)
• heap_endscan(scan) … frees up scan struct
• res = HeapKeyTest(tuple,…,nkeys,keys) 
… performs ScanKeys tests on tuple … is it a result tuple?
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❖ Scanning in PostgreSQL (cont)

typedef HeapScanDescData *HeapScanDesc;

typedef struct HeapScanDescData
{
// scan parameters
Relation rs_rd; // heap relation descriptor
Snapshot rs_snapshot; // snapshot … tuple visibility
int rs_nkeys; // number of scan keys
ScanKey rs_key; // array of scan key descriptors
…
// state set up at initscan time
PageNumber rs_npages; // number of pages to scan
PageNumber rs_startpage; // page # to start at
…
// scan current state, initally set to invalid
HeapTupleData rs_ctup; // current tuple in scan
PageNumber rs_cpage; // current page # in scan
Buffer rs_cbuf; // current buffer in scan
…
} HeapScanDescData;

COMP9315 21T1 ♢ Scanning ♢ [15/16]
<< ∧ ❖ Scanning in other File Structures Above examples are for heap files • simple, unordered, maybe indexed, no hashing Other access file structures in PostgreSQL: • btree, hash, gist, gin • each implements: ◦ startscan, getnext, endscan ◦ insert, delete  (update=delete+insert) ◦ other file-specific operators COMP9315 21T1 ♢ Scanning ♢ [16/16] Produced: 28 Feb 2021