Red-Index | Auto-indexer of repositories and cogs
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kandi X-RAY | Red-Index Summary
Auto-indexer of repositories and cogs
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- Process all the cog
- Read the info json
- Check the cog package
- Check repo info
- Return sha1 hash of url
- Make the error log for repos
- Populates the list of cogs
- Clean url
- Return sha1 digest of url
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QUESTION
I try to understand indexes in Azure Synapse and I'm a bit confused by some of them.
Regarding the Clustered Columnstore Index, I've a feeling that it works a bit like Apache Parquet, with row groups and column chunks inside. In heap tables the data is not indexed, so it seems pretty clear too.
But what about the clustered and nonclustered indexes? The documentation defines them as:
Clustered indexes may outperform clustered columnstore tables when a single row needs to be quickly retrieved. For queries where a single or very few row lookup is required to perform with extreme speed, consider a clustered index or nonclustered secondary index. The disadvantage to using a clustered index is that only queries that benefit are the ones that use a highly selective filter on the clustered index column. To improve filter on other columns, a nonclustered index can be added to other columns. However, each index that is added to a table adds both space and processing time to loads.
Here are my questions:
- Does it mean they're more like the indexes from SQL Server? I mean, the clustered index would order the data by one column and store it as rows? And the non clustered would be an extra sorted index storing only references to the rows?
- If my assumption about row-based format is correct, does it mean the clustered index is not performant for the analytical queries, doesn't it?
- What happens if we create a table with both Columnstore and Clustered Indexes? The data is duplicated, once for the columnar format, once for the row format?
Some links I found on that topic, but still have some doubts whether they apply to Synapse:
- https://crmchap.co.uk/understanding-table-distribution-index-types-in-azure-synapse-analytics/
- https://www.sqlservercentral.com/articles/introduction-to-indexes-part-2-%e2%80%93-the-clustered-index
- https://www.sqlservercentral.com/articles/introduction-to-indexes-part-3-%E2%80%93-the-nonclustered-index
- https://docs.microsoft.com/en-us/sql/t-sql/statements/create-table-azure-sql-data-warehouse?toc=%2Fazure%2Fsynapse-analytics%2Fsql-data-warehouse%2Ftoc.json&bc=%2Fazure%2Fsynapse-analytics%2Fsql-data-warehouse%2Fbreadcrumb%2Ftoc.json&view=azure-sqldw-latest&preserve-view=true#rowstore-table-heap-or-clustered-index
ANSWER
Answered 2021-May-05 at 14:50Bartosz,
Does it mean they're more like the indexes from SQL Server? I mean, the clustered index would order the data by one column and store it as rows? And the non clustered would be an extra sorted index storing only references to the rows?
You are correct on clustering and non clustering definition - with a slight twist. It is similar to traditional SQL Server and that the leaf of cluster is the actual data row. In summary, the physical organization of data rows for Synapse/pdw will be
Clustered columnstore - data is not sorted and row segments can have overlapping min-max values
Clustered columstore with order by - data is sorted, hence the row segments will not have overlapps and skipping will optimal
Heap - which is row format
Clustered index this is SQL Server clustered index where lead/data portion is sorted.
If my assumption about row-based format is correct, does it mean the clustered index is not performant for the analytical queries, doesn't it?
Clustered index will be performant if your query selects a set of values are sequential. for example - select * from table where year between 2005 and 2007
. Row/Heap tables are efficient if your projection/select includes all or most of the columns of the table. Columnstore organization is efficient if have wide tables and select a handful of columns.
What happens if we create a table with both Columnstore and Clustered Indexes? The data is duplicated, once for the columnar format, once for the row format? If you have a columstore index, you wont be able to create a clustered index.
QUESTION
Where can I find a good representation of the of how data is stored in pages and how the B tree is constructed for a multi-column index (specifically for SQL server, but not necessarily)?
I'm referring to something like what you see in https://docs.microsoft.com/en-us/sql/relational-databases/reading-pages?view=sql-server-ver15 (for single column) but extended for multi-columns.
Another example for single column index:
Thanks.
...ANSWER
Answered 2020-Jul-21 at 12:49The index key values are sorted first by the first key column, then by the second key column, and then yt's exactly the same, except with additional columns on the non-leaf nodes. So if the first key column is a number, and the second the name of an animal, the non-leaf pages might have ranges like:
QUESTION
I have a table called 'GameTransactions'. It is critical for the table to work well in terms of performance (The table will have millions of records when the site is going to be operational). I thought to index it. The columns that I used for the columns are:
...ANSWER
Answered 2020-Jun-07 at 10:55Use EXISTS
instead of COUNT
to conditionally insert the row. This will be more efficient since a count is not needed. Make sure the index is unique to ensure duplicates are not possible.
Use >=
instead of >
for the timestamp criteria so that 2 sessions with the same timestamp don't both insert the same row, although one would err if a unique index or constraint exists.
Furthermore, consider removing NOLOCK
to ensure concurrent sessions don't insert rows for the same UserID/TransactionID/ProviderID withing the TransactionTimeStamp date range. I suggest SERIALIZABLE
for this purpose. Example DDL below with the query encapsulated in a stored procedure below, leveraging the primary key index for both performance and data integrity.
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You can use Red-Index like any standard Python library. You will need to make sure that you have a development environment consisting of a Python distribution including header files, a compiler, pip, and git installed. Make sure that your pip, setuptools, and wheel are up to date. When using pip it is generally recommended to install packages in a virtual environment to avoid changes to the system.
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