In this post I will briefly summarize why analytic (OLAP) workloads perform better on columnar (aka column-oriented) databases as opposed to traditional row-based (aka row-oriented) databases.
- Storage Organization
- Vectorized Query Execution
- CPU Cache Friendly
- Late Materialization
Analytic workloads comprise of operations like scans, joins, aggregations etc. These operations are concerned with data retrieval, some computations over the values stored in table cells, and predicate evaluation. Such operations typically touch a very large number of rows but only a few columns in the table. As far as I understand, this is the fundamental difference between analytic queries and OLTP-style operations. Here are some examples differentiating the two types of operations.
|Analytic Operations||OLTP Operations|
|SELECT SUM(SALARY) FROM EMPLOYEE||SELECT * FROM EMPLOYEE WHERE SSN=123456|
|SELECT ORDERVALUE FROM ORDERS WHERE STATE=’CA’||INSERT INTO ORDERS VALUES ( …. ) WHERE ORDERNUM=|
|SELECT COUNT(*) FROM EMPLOYEE WHERE SALARY > 150000 AND SALARY < 300000||UPDATE EMPLOYEE SET SALARY=SALARY+15000 WHERE SSN=123456;|
|SELECT MIN(ORDERVALUE) FROM ORDERS WHERE STATE=’CA’|
|SELECT COLUMN1 + COLUMN2 AS COLUMN3 FROM FOO|
|SELECT FIRST_NAME, LAST_NAME FROM EMPLOYEE WHERE RATING=’EE’|
Now let’s talk about some of the factors that work in favor of column stores for efficient analytical queries.
1. Organization of Data
The fundamental (and most obvious) difference between column stores and row stores is the way they organize and store table data.
Row-oriented databases store data on a row-by-row basis. Each row has its data stored together (contiguously) in-memory or on-disk or both. Thus its easier to grab some/all columns of a “particular row” given its ROWID. So in a single seek, row’s data can be loaded from disk into memory.
However, if the operation requires to access only a handful of columns from a very large number of rows in the table (the case of analytic or data warehouse queries), row stores tend to become less efficient since we have to walk the row, skip over the columns not needed, extract the value from relevant column of interest and move on to the next row.
In columnar databases, values of a particular column are stored separately or individually — contiguous layout for values of a given column in-memory or on-disk. If the query touches only few columns, it is relatively faster to load all values of a “particular column” into memory from disk in fewer I/Os and further into CPU cache in fewer instructions.
In other words, column stores are capable of accessing values of a particular column independently without bothering about the other columns in the table which may not even be needed by the query plan. Thus in columnar storage format, there is no need to read (and then skip/discard) columns that are not needed and this provides better utilization of available I/O and CPU-memory bandwidth for analytical queries.
2. Vectorized Query Execution
The columnar storage format allows other efficient techniques to be built on top of it to deliver high performance for analytical or data warehouse type of queries.
Columnar databases are capable of doing faster predicate evaluation by exploiting data level parallelism through SIMD instructions. Multiple column values can be operated (+, =, <, > etc) on in a single CPU instruction using SIMD technology.
For example, with 128 bit SIMD register, predicate evaluation using “=” operator can be done on 8 2-byte column values in a single instruction — data level parallelism. Not only the evaluation is faster, but a single instruction can also load multiple column values into SIMD register from columnar in-memory storage format.
Example Query: SELECT COUNT(*) FROM EMPLOYEE WHERE AGE>40 AND AGE<55
AGE column can be efficiently bit-packed into fixed-width 1 byte numeric value instead of using a regular 4 or 8 byte numeric values. Thus 16 values from AGE column can be evaluated against the provided predicates in a single instruction.
SQL SUM, AVG, COUNT, MIN, MAX can all be accelerated using SIMD technology.
3. CPU Cache Friendly
The columnar storage format allows better utilization of CPU cache since cache lines are full of related values (same column) that are needed by query executor to run some operations (evaluation, computation etc). In other words, only data that really needs to be examined is brought into cache. Thus looping through values of a column packed into columnar format is faster.
This is completely different from tuple based iterator in conventional row stores where a tuple consists of values from multiple columns and operator might need value only from one column. Thus cache tends to contain mixed data.
4. Late Materialization
Any query eventually needs to send back the result-set (tuples) to the end user. This should happen regardless of what type (column store or row store) of data store is being used.
For example, the query should ultimately send back tuples <first_name, last_name, age> to the user even though the columns are stored separately in a column store.
SELECT FIRST_NAME, LAST_NAME, AGE FROM EMPLOYEE WHERE AGE>40 AND AGE<60
- AGE column can be scanned first to do efficient (SIMD scans) predicate evaluation and filter out the values that don’t pass the condition.
- The output of SIMD scan (typically a mask or bit-vector) can then be used to scan through the FIRST_NAME and LAST_NAME columns and pick the values corresponding to matched positions in AGE column and keep forming the resultant tuples that can be sent back to client.
What did we do above?
- The query plan for this query touches 3 columns — FIRST_NAME, LAST_NAME and AGE with predicate evaluation needed on AGE column.
- There is typically one decision that needs to be made — when do we form a tuple? In other words, when do we materialize values from different columns into a tuple?
- Two directions can be taken here:
- As soon as query plan touches a column (SELECT list or for WHERE clause or any other operator), the column value is added to the tuple. Note that word “touches” is very important here.
- The operator evaluation or any other processing hasn’t happened yet on the column value. The fact that it is needed by the query plan for something will dictate the addition of column value to the tuple.
- We end up forming tuples that eventually get discarded simply because operator evaluation failed on one of the columns.
<John, Smith, 35>, <Ray, Lane, 45>. The first tuple will get discarded later on when WHERE clause is executed on AGE column value.
Instead, why not delay the materialization of column values and ensure we only form relevant tuples — tuples that are actually part of result-set.
- Take advantage of the columnar format and process the individual column first.
- For example, in a tight for-loop AGE column was scanned first to do predicate evaluation on all the column values.
- The output of SIMD scan was then fed through the scan of FIRST_NAME and LAST_NAME columns to pick only the values that are needed for result-set and form relevant tuples.
How did Late Materialization help us?
- We formed only relevant tuples and that too much later during execution. No CPU was wasted on forming tuples upfront without knowing if a particular tuple might eventually be a part of result-set or not.
- This is more efficient utilization of CPU-memory bandwidth.
- Leveraged the storage format (columnar) and applied vectorized query processing during predicate evaluation on a particular column. If the column values are going to be materialized into tuples sooner in the query plan, then we lose the opportunity to use vectorized instructions on a column simply because after materialization the data is no longer columnar and we need to work with tuples.
- Because materialization is done later in the plan, we quickly loop through the values of a column and applied necessary filters.
- This is far more efficient than a tuple based iterator that we need to work with in the case of early materialization since for each tuple, we feed the right column value (AGE) into the operator (WHERE) and decide if the tuple is needed or not. The tuple based iteration is not so “CPU-cache friendly” because the tuple contains “N” fields and all we need is just one field to pass to the operator repeatedly (in a loop)
- There are more advantages of Late Materialization in the context of Compression and they will be discussed in the next section.
Data stored in database tables is usually compressed to optimize disk space storage and I/O during query processing — reduced number of bytes (compressed) will be read off disk into memory and down through the storage hierarchy.
Compression algorithms operate better if the input data is somewhat related (less entropy) and this gives better compression ratios. Columnar format can take advantage of this fact and each column can be compressed individually with a compression scheme most suitable for that column. This advantage is however not available in row stores since a row contains data from multiple different types of columns. This doesn’t imply that compression algorithms are guaranteed to give better compression ratios on columnar data. It’s just that related (all column values) data has the potential to be highly compressible.
Different factors can be taken into account when deciding upon a compression scheme to be used for a particular column. Factors like column cardinality, data type, sorted or not etc are important column-level characteristics that can be used to decide the compression method.
The focus is not really on getting best compression ratios. Instead, we should think about schemes that enable faster processing (predicate evaluation, decompression etc) on column values. Columnar format allows us to use simple and efficient compression methods like Run Length Encoding (RLE), Dictionary Encoding, bit-vector encoding, delta encoding etc that may not give the best compression ratios but allow faster decompression.
As an example, dictionary encoding is usually a good choice if the column has low cardinality — limited number of possible unique values. The encoded values (dictionary indexes) can be stored in fewer bits. COUNTRY column in a table can be efficiently represented using dictionary encoding.
There are some advantages of using such light-weight compression methods:
- Compliment vectorized query processing by potentially packing the data (uncompressed column values) into fixed-width values in an array (compressed column values).
- SIMD instructions can then work on this well packed data and process the compressed column values.
- The encoding schemes mostly allow us to work on compressed column values and process them before decompressing everything upfront. This way we can decompress only the needed values — values that passed predicate filters.
- Obviously raw column data might already be fixed width — for example AGE column might be internally represented using an array of 4 byte integers. So without doubt, SIMD techniques will be effective on raw column data as well in this case.
- However, if we compress the AGE column using bit-packing (1 byte is enough to represent any value in AGE column), the processing actually becomes much faster because the number of column values that can be loaded (and parallelly processed) into SIMD register (typically 128 bits) are far more if the column is compressed.
Late materialization becomes more effective since the idea of processing the column values before forming tuples actually allows us to work on compressed columnar data. Once a given column has been processed and we need to form tuples, we can decompress only the necessary values.
We want to find out all the states/regions in England that had more than half million in sales.
SELECT STATE FROM SALES WHERE COUNTRY=’ENGLAND’ AND SALEVALUE>500000
- COUNTRY column can be stored as dictionary encoded when the table was loaded with data.
- Let’s say SALEVALUE column is internally stored in 4-byte fixed width values (4 billion max sales). I feel there is no harm in storing this column as raw uncompressed.
- So the query executor will first probe the dictionary to get fixed-width encoded value for predicate ‘ENGLAND’. Say this is 4.
- Compressed COUNTRY column can be quickly scanned using vectorized instructions.
- Multiple (16 if each encoded value is stored in 1 byte) encoded column values can be loaded into a 128 bit SIMD register and parallelly compared with value 4.
- SALEVALUE column can then be scanned with vectorized instructions.
- 4 column values can be loaded into a 128 bit SIMD register and parallelly compared with value 500000.
- Bitwise AND can be done on the output of two SIMD scans (COUNTRY and SALEVALUE).
- The output of SIMD scans is typically a bit vector or mask indicating where the predicate passed or failed.
- All we are trying to do now is AND the vectors and find where both predicates passed or failed
- bit_vector R = SIMD_SCAN_COUNTRY & SIMD_SCAN_SALEVALUE
- Now we can use the bit-vector R and scan through the STATE column to pick a column value at position “N” if “Nth” bit is set in R.
With this I would like to conclude the post. I agree that the post doesn’t dig into specific details about using compression, vectorized processing, query plan etc. However, the goal of this post is to just give a brief overview. Subsequent posts will take a deep dive into individual topics.