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Questions tagged [database-scan]

60 questions
231
votes
4 answers

Why does PostgreSQL perform sequential scan on indexed column?

Very simple example - one table, one index, one query: CREATE TABLE book ( id bigserial NOT NULL, "year" integer, -- other columns... ); CREATE INDEX book_year_idx ON book (year) EXPLAIN SELECT * FROM book b WHERE b.year > 2009 gives…
Alex Vayda
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4 answers

HBase (Easy): How to Perform Range Prefix Scan in hbase shell

I am designing an app to run on hbase and want to interactively explore the contents of my cluster. I am in the hbase shell and I want to perform a scan of all keys starting with the chars "abc". Such keys might inlcude "abc4", "abc92", "abc20014"…
David Williams
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5 answers

sql statistics io scan count explanation

Simple question, but I haven't found a good explanation on google. When using Set Statistics IO ON, the logical reads and scan count is provided in the message window of management studio. If I have: tblExample, scan count 5, logical reads 20 What…
Paulj
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1 answer

DynamoDB Scan with filter, matching 'is-in-set' conditions

I'm trying to use a Scan operation on a DynamoDB table to match items with given 'tags'. The tags for an item are stored in a single set attribute. E.g.: machine-1: tags = "windows", "iis", "64bit" machine-2: tags = "windows", "fs" Now, I have a…
Kieren Johnstone
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10
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1 answer

HBase Scan with Multiple Ranges

I have a HBase table, and I need to get the result from several ranges. For example, I may need get data from different ranges like row 1-6, 100-150,..... I know that for each scan, I can define the start row and stop row. But if I have 6 ranges, I…
Cheng Chen
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10
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2 answers

HBase Scan Performance

I am performing a range scan that is giving me 500k records. If I set scan.setCaching(100000) it took less than one second, but if scan.setCaching(100000) is not set it took nearly 38 sec. If I set scan.setBlockCache(false) and…
Hanish Bansal
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10
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2 answers

Should I use prefixfilter or rowkey range scan in HBase

I don't know why it's very slow if I use prefixfilter to query. Can someone explain which is the best way to query HBase, thanks. hbase(main):002:0> scan…
zhukunpeng
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7
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2 answers

How to apply several QualifierFilter to a row in HBase

we would like to filter a scan on a HBase table with two QualifierFilters. Means we would like to only get the rows of the table which do have a certain column 'col_A' AND (!) a certain other column 'col_B'. Our current approach looks like…
Henrik
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6
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1 answer

Does HBase uses a primary index?

How does HBase performs a lookup and retrieves a record ? e.g., what is the equivalent in HBase for the RDBMS's B-Trees? [EDIT] I understand how HBase resolves the -ROOT-, and .META. tables to find out which region holds the data. But how is the…
David
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5
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Is there a predictable order when scanning/scrolling an elastic search index?

When scanning an ElasticSearch index it is not possible to apply any sorting according to the documentation. But is there any definition at all about the order of the results during this process? If yes, is it predictable? Background info: I need to…
peterp
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4
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1 answer

Why oracle CHOOSE INDEX RANGE SCAN over FAST FULL INDEX SCAN

I have read some documentation about indexes, I did some examples and now I have some doubts. I create a table and insert random values, (A column has unique values) column A NOT NULL I create an index on A, B, C. (B-TREE) SELECT COUNT(*) FROM…
Samardj
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4
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1 answer

hbase shell command - scan and Filter

I am pretty new to Hadoop and hbase. Can anybody help me with the scan and filter command. I was trying to pull the records in hbase for a range of rows and for a particular drug, using Filter. When I am giving this shell command: scan 'Mytable',…
Nimisha
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2 answers

Hbase Scan Filter for Records that are Missing Column or the Column Value is null

I have 3 cols {col1, col2, col3} in a col family (CF). I'd like to write a scan such that I select all rows that have col1='val1' and col2='val2' and (col3 is missing or col3 is null). Doing this in Java - sorry I'm totally new to hbase ;-)
hba
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2 answers

How to store data in HBase for efficient fetching with Partial Key scanning?

My key has three components: num, type, name The 'type' is only of two kinds A and B while num can have more values e.g. 0,1,2..,30 I have to fetch data with respect to num and type i.e. fetch all rows which have keys with the specified num and…
Monis Iqbal
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HBase: How to fetch rows using row key range if row key is stored as byte array value

We are using HBase for storing data which is sqooped from oracle to hdfs. Here we designed the row key as byte array value. Row key is framed with composite key like (Md5(schema name).getBytes() + Md5(date (format = yyyy-mm-dd)).getBytes() +…
GHK
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