fetch()
Here’s how to use fetch() function.
cs .fetch()
Description:
Fetch one or more records from a cursor/cluster cursor.
Syntax:
cs.fetch(n;x)
Note:
Only one of the parameters n and x is valid. The function fetches n records when n is valid, or fetches records continuously till the value of expression x (Here x doesn’t return a boolean value) changes or becomes true (Here x returns a boolean value) when x is valid, and then returns them as a sequence/record sequence/table sequence or returns null if the cursor has already been fetched out. It is often used to retrieve a large amount of data in batches. When both n and x are omitted, the function returns all records in a cursor and closes cursor.
Parameter:
|
cs |
A cursor/cluster cursor |
|
n |
An integer |
|
x |
Grouping expression, according to which cs is sorted. With x, n will be ignored |
Option:
|
@0 |
Won’t actually fetched out the selected data from the cursor. The option enables an action functionally equivalent to copying the data; it doesn’t support parameter x. |
|
@x |
Close the cursor after data is fetched. |
Return value:
A sequence/record sequence/table sequence
Example:
|
|
A |
|
|
1 |
=demo.cursor("select * from EMPLOYEE order by SALARY desc") |
Return a cursor of retrieved data sorted by SALARY. |
|
2 |
=A1.fetch@0(3) |
Select top 3 highest-paid employees. |
|
3 |
=A1.derive(interval@y(BIRTHDAY,HIREDATE):EntryAge, age(HIREDATE):WorkAge) |
With @0 option used in A2, A3 fetches data from a complete cursor |
|
4 |
=A1.fetch() |
|
|
5 |
=demo.cursor("select * from EMPLOYEE order by SALARY desc") |
|
|
6 |
=A1.fetch(;SALARY) |
Stop data retrieval when SALARY change to select a group of highest- and equally-paid employees.. |
|
7 |
=A1.fetch() |
Return the remaining records in the cursor. |
|
8 |
=demo.cursor("select * from EMPLOYEE order by SALARY desc") |
|
|
9 |
=A8.fetch@x(3) |
Close the cursor after data is fetched. |
|
10 |
=demo.cursor("select * from EMPLOYEE order by SALARY desc") |
|
|
11 |
=A10.fetch(3;SALARY==13000) |
Stop data retrieval when the value of the expression is true
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|
|
A |
|
|
1 |
[192.168.0.110:8281,192.168.18.143:8281] |
A sequence of nodes |
|
2 |
=file("emp.ctx":[1,2], A1) |
1.emp.ctx on node 192.168.0.110 holds a part where EID is from 1 to 250 and 2.emp.ctx on node 192.168.18.143 holds another part where EID is from 251 to 500 |
|
3 |
=A2.open() |
Open a cluster homo-name files group |
|
4 |
=A3.cursor() |
Return a cluster cursor |
|
5 |
=A4.fetch() |
Fetch records form the cluster cursor |
Related function:
ch .fetch()
Description:
Fetch and store the existing data in a channel.
Syntax:
ch.fetch(f)
Note:
The function fetches and stores the existing data in channel ch; write result of data retrieval to bin file f when parameter f is present. It is a result set generation function.
Parameter:
|
ch |
Channel |
|
f |
A bin file |
Return value:
Channel
Example:
|
|
A |
|
|
1 |
=demo.cursor("select * from SALES") |
|
|
2 |
=channel() |
Create a channel |
|
3 |
=channel() |
|
|
4 |
=A2.select(ORDERID>100) |
Attach ch.select() operation to the channel |
|
5 |
=A3.select(ORDERID>100) |
|
|
6 |
=A2.fetch() |
Attach ch.fetch() function that returns the final result to A2’s channel while retaining the exisitng data in the channel |
|
7 |
=A3.fetch("sales.btx") |
Attach ch.fetch() function that gets the final result set to A3’s channel, and define writing result to bin file sales.btx in the main directory |
|
8 |
=A1.push(A2,A3) |
Push A1’s data in into the channel |
|
9 |
=A1.fetch() |
|
|
10 |
=A2.result() |
Get the result of performing the operation from the channel |
mcs .fetch()
Description:
Get records from a multicursor.
Syntax:
mcs.fetch()
Note:
The function gets all records from a multicursor. The order of the resulting records could be different from their orginal order.
Parameter:
|
mcs |
Multicursor |
Return value:
Record sequence
Example:
|
|
A |
|
|
1 |
=file("D:/txt_files/orders.txt").cursor@m() |
Return a multicursor |
|
2 |
=A1.fetch() |
Fetch records from a multicursor |
|
3 |
=file("D://tb1.txt").import() |
tb1.txt contains 10w rows |
|
4 |
=A3.cursor() |
Generate an ordinary cursor |
|
5 |
=A4.fetch(15000) |
Fetch the first 15000 rows
|
|
6 |
=A3.cursor@m(10) |
Generate a multicursor |
|
7 |
=A6.fetch(15000) |
Fetch the first 15000 rows
Here the fetched records are different from those in A5. With a multicursor, data is fetched from each cursor and then unioned; while with an ordinary cursor, data is sequentially fetched |
