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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
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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
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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 dont find)
average case: read half of the pages (b/2)
Page Costs: Costavg = b/2 Costmin = 1 Costmax = b
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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
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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);
}

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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
intcurPID;// current pid
intcurTID;// 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);
}

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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
}

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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);

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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
Relationrs_rd;// heap relation descriptor
Snapshotrs_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
PageNumberrs_npages;// number of pages to scan
PageNumberrs_startpage; // page # to start at

// scan current state, initally set to invalid
HeapTupleData rs_ctup;// current tuple in scan
PageNumberrs_cpage; // current page # in scan
Bufferrs_cbuf;// current buffer in scan

} HeapScanDescData;

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<< Scanning in other File StructuresAbove 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