Postgresql sort array

Postgresql sort array DEFAULT

Summary: in this tutorial, you will learn how to use the PostgreSQL aggregate function to return an array from a set of input values.

Introduction to PostgreSQL function

The PostgreSQL function is an aggregate function that accepts a set of values and returns an array in which each value in the set is assigned to an element of the array.

The following shows the syntax of the function:

Code language:CSS(css)

The accepts an expression that returns a value of any type which is valid for an array element.

The clause is an optional clause. It specifies the order of rows processed in the aggregation, which determines the order of the elements in the result array.

Similar to other aggregate functions such as , , , , and , the is often used with the clause.

PostgreSQL function examples

We will use the , , and tables from the sample database for the demonstration.

Using PostgreSQL function without the clause example

The following example uses the function to return the list of film title and a list of actors for each film:

Code language:PostgreSQL SQL dialect and PL/pgSQL(pgsql)

Here is the partial output: 

As you can see, the actors in each film are arbitrarily ordered. To sort the actors by last name or first name, you can use the clause in the function.

Using PostgreSQL function with the ORDER BY clause example

This example uses the function to return a list of films and a list of actors for each film sorted by the actor’s first name:

Code language:PostgreSQL SQL dialect and PL/pgSQL(pgsql)

The following shows the partial output:

You can sort the actor list for each film by the actor’s first name and last name as shown in the following query:

Code language:PostgreSQL SQL dialect and PL/pgSQL(pgsql)

This picture shows the partial output of the query:

In this tutorial, you have learned how to use the PostgreSQL function to return an array from a set of input values.

Sours: https://www.postgresqltutorial.com/postgresql-aggregate-functions/postgresql-array_agg-function/

Sorting array elements

If you're looking for a solution which will work across any data-type, I'd recommend taking the approach laid out at YouLikeProgramming.com.

Essentially, you can create a stored procedure (code below) which performs the sorting for you, and all you need to do is pass your array to that procedure for it to be sorted appropriately.

I have also included an implementation which does not require the use of a stored procedure, if you're looking for your query to be a little more transportable.

Creating the stored procedure

Sorting array values (works with any array data-type)

Sorting array values without a stored procedure

In the following query, simply replace with your array or query which returns an array:

... or ...

answered Jul 8 '13 at 21:23

Joshua BurnsJoshua Burns

7,40033 gold badges4343 silver badges5959 bronze badges

Sours: https://stackoverflow.com/questions/2913368/sorting-array-elements
  1. Gerry dress
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  3. Wilcom digitizer
  4. Father pfleger

F.15. intarray

The module provides a number of useful functions and operators for manipulating one-dimensional arrays of integers. There is also support for indexed searches using some of the operators.

F.15.1. Functions and Operators

Table F-7. Functions

FunctionReturn TypeDescriptionExampleResult
number of elements in array
sort array — must be or
sort in ascending order
sort in ascending order
sort in descending order
remove adjacent duplicates
index of first element matching (0 if none)
portion of array starting at position , elements
portion of array starting at position
make single-element array

Table F-8. Operators

OperatorReturnsDescription
overlap — if arrays have at least one common element
contains — if left array contains right array
contained — if left array is contained in right array
number of elements in array
index (same as function)
push element onto array (add it to end of array)
array concatenation (right array added to the end of left one)
remove entries matching right argument from array
remove elements of right array from left
union of arguments
union of arrays
intersection of arrays
if array satisfies query (see below)
if array satisfies query (commutator of )

(Before PostgreSQL 8.2, the containment operators and were respectively called and . These names are still available, but are deprecated and will eventually be retired. Notice that the old names are reversed from the convention formerly followed by the core geometric datatypes!)

The containment operators and are approximately equivalent to PostgreSQL's built-in operators of the same names, except that they work only on integer arrays while the built-in operators work for any array type. An important difference is that 's operators do not consider an empty array to be contained in anything else. This is consistent with the behavior of GIN-indexed queries, but not with the usual mathematical definition of containment.

The and operators test whether an array satisfies a query, which is expressed as a value of a specialized data type . A query consists of integer values that are checked against the elements of the array, possibly combined using the operators (AND), (OR), and (NOT). Parentheses can be used as needed. For example, the query matches arrays that contain 1 and also contain either 2 or 3.

F.15.2. Index Support

provides index support for the , , , and operators, as well as regular array equality.

Two GiST index operator classes are provided: (used by default) is suitable for small- to medium-size data sets, while uses a larger signature and is more suitable for indexing large data sets (i.e., columns containing a large number of distinct array values). The implementation uses an RD-tree data structure with built-in lossy compression.

There is also a non-default GIN operator class supporting the same operators.

The choice between GiST and GIN indexing depends on the relative performance characteristics of GiST and GIN, which are discussed elsewhere. As a rule of thumb, a GIN index is faster to search than a GiST index, but slower to build or update; so GIN is better suited for static data and GiST for often-updated data.

F.15.3. Example

-- a message can be in one or more "sections" CREATE TABLE message (mid INT PRIMARY KEY, sections INT[], ...); -- create specialized index CREATE INDEX message_rdtree_idx ON message USING GIST (sections gist__int_ops); -- select messages in section 1 OR 2 - OVERLAP operator SELECT message.mid FROM message WHERE message.sections && '{1,2}'; -- select messages in sections 1 AND 2 - CONTAINS operator SELECT message.mid FROM message WHERE message.sections @> '{1,2}'; -- the same, using QUERY operator SELECT message.mid FROM message WHERE message.sections @@ '1&2'::query_int;

F.15.4. Benchmark

The source directory contains a benchmark test suite. To run:

cd .../bench createdb TEST psql TEST < ../_int.sql ./create_test.pl | psql TEST ./bench.pl

The script has numerous options, which are displayed when it is run without any arguments.

Sours: https://www.postgresql.org/docs/8.4/intarray.html
Learn PostgreSQL Tutorial - Full Course for Beginners

How to preserve the original order of elements in an unnested array?

in Postgres 9.4 or later

The query can now simply be:

Or, applied to a table:

Details:

About the implicit join:

db<>fiddle here
Old sqlfiddle

Postgres 9.3 or older - and more general explanation

For a single string

You can apply the window function to remember the order of elements. However, with the usual you get numbers according to the sort order, not the original position in the string.

You could simply omit to get the position "as is":

Performance of degrades with long strings. scales better:

However, while this normally works and I have never seen it break in simple queries, Postgres asserts nothing as to the order of rows without an explicit .

To guarantee ordinal numbers of elements in the original string, use (improved with comment by @deszo):

For a table of strings

Add to the clause ...

Demo table:

I use as ad-hoc substitute for a primary key. If you have one (or any unique column) use that instead.

This works without any distinct ID:

Answer to question

Sours: https://dba.stackexchange.com/questions/27279/how-to-preserve-the-original-order-of-elements-in-an-unnested-array

Array postgresql sort

9.18. Array Functions and Operators

append an element to the end of an arrayconcatenate two arraysreturns the number of dimensions of the arrayreturns a text representation of array's dimensionsreturns an array initialized with supplied value and dimensions, optionally with lower bounds other than 1returns the length of the requested array dimensionreturns lower bound of the requested array dimensionappend an element to the beginning of an arrayremove all elements equal to the given value from the array (array must be one-dimensional)replace each array element equal to the given value with a new valueconcatenates array elements using supplied delimiter and optional null stringreturns upper bound of the requested array dimensionreturns the total number of elements in the array, or 0 if the array is emptysplits string into array elements using supplied delimiter and optional null stringexpand an array to a set of rows1 2 (2 rows)expand multiple arrays (possibly of different types) to a set of rows. This is only allowed in the FROM clause; see Section 7.2.1.41 foo 2 bar NULL baz (3 rows)
Sours: https://www.postgresql.org/docs/9.4/functions-array.html
Algorithms: Sort An Array with Comparator

Manipulating arrays with PostgreSQL

Before we move on to manipulating array values stored in the database, it is useful to first learn the basics.

Representing a list

We have to specify the type. Else postgres would not know the type since it looks like a string.

Or we could also use the style

ing an array

So why doesn’t the following work?

expects that the data source is:

  • a set of rows
  • OR points to a set of word (table)

returns an atomic value. Which is why it works with just but fails when is used.

Convert arrays into rows using

Convert rows into arrays using

Watch out. We are not using . We are using .

To summarize:

  • converts array into a series of rows.
  • converts a bunch of rows into an array.
  • used to represent/define an array.

Concatenate arrays using

We coerced the argument into text arrays to indicate to what data types it was dealing with. If we used the initializer, this would not be required. Example below.

Notice that the final array contains twice. We would love to remove duplicates. There are two ways to do it and we'll explore both below.

Removing duplicates from an array using

This method works with integers, text, float, etc

helps remove duplicates from all data types. But it requires rows as input. Not arrays.

  • So first convert an array to rows using
  • Pass it to DISTINCT
  • And then if you still require an array as output, use

And we have columns. But we need an array back as output. So convert using

What if we want it sorted? Use

would sort by the first column (We could starting with here). In this case we are working with on-the-fly arrays, without giving them column names. So we use column number references.

Want to try this with a float array? Let's do it.

Removing duplicates from integer arrays using and

Caveat: This method only works for integer arrays (yeah so no floats too).

and are functions available in the extension. Enable the extension in your database by running the following statement.

Try using the function.

Oops. Does not work. only works on a sorted list. So use the function to sort it first.

I haven't yet benchmarked in detail which method works the fastest for sorting integers. I used both methods as a part of another benchmarking to update 100k rows (coming soon). For now, all I can say is, both aren't slow. I'll try a more detailed benchmarking another day.

What else?

These are essentials that I find useful to know. There are a lot more array functions that Postgres supports. Go check it out.

I'll be sharing more from my notes on PostgreSQL. Subscribe to get updates.

Sours: https://dev.to/johncarrot/manipulating-arrays-with-postgres-ak1

You will also be interested:

F.12. intarray

The module provides a number of useful functions and operators for manipulating one-dimensional arrays of integers. There is also support for indexed searches using some of the operators.

F.12.1. Functions and Operators

Table F-7. Functions

FunctionReturn TypeDescriptionExampleResult
number of elements in array
sort array — must be or
sort in ascending order
sort in ascending order
sort in descending order
remove adjacent duplicates
index of first element matching (0 if none)
portion of array starting at position , len
portion of array starting at position
make single-element array

Table F-8. Operators

OperatorReturnsDescription
overlap — if arrays have at least one common element
contains — if left array contains right array
contained — if left array is contained in right array
number of elements in array
index (same as function)
push element onto array (add it to end of array)
array concatenation (right array added to the end of left one)
remove entries matching right argument from array
remove elements of right array from left
union of arguments
union of arrays
intersection of arrays
if array satisfies query (see below)
if array satisfies query (commutator of )

(Before PostgreSQL 8.2, the containment operators @> and <@ were respectively called @ and ~. These names are still available, but are deprecated and will eventually be retired. Notice that the old names are reversed from the convention formerly followed by the core geometric datatypes!)

The and operators test whether an array satisfies a query, which is expressed as a value of a specialized data type . A query consists of integer values that are checked against the elements of the array, possibly combined using the operators (AND), (OR), and (NOT). Parentheses can be used as needed. For example, the query matches arrays that contain 1 and also contain either 2 or 3.

F.12.2. Index Support

provides index support for the , , , and operators, as well as regular array equality. The implementation uses an RD-tree data structure with built-in lossy compression.

Two GiST index operator classes are provided: (used by default) is suitable for small and medium-size arrays, while uses a larger signature and is more suitable for indexing large arrays.

There is also a non-default GIN operator class .

The choice between GiST and GIN indexing depends on the relative performance characteristics of GiST and GIN, which are discussed elsewhere. As a rule of thumb, a GIN index is faster to search than a GiST index, but slower to build or update; so GIN is better suited for static data and GiST for often-updated data.

F.12.3. Example

-- a message can be in one or more "sections" CREATE TABLE message (mid INT PRIMARY KEY, sections INT[], ...); -- create specialized index CREATE INDEX message_rdtree_idx ON message USING GIST (sections gist__int_ops); -- select messages in section 1 OR 2 - OVERLAP operator SELECT message.mid FROM message WHERE message.sections && '{1,2}'; -- select messages in sections 1 AND 2 - CONTAINS operator SELECT message.mid FROM message WHERE message.sections @> '{1,2}'; -- the same, using QUERY operator SELECT message.mid FROM message WHERE message.sections @@ '1&2'::query_int;

F.12.4. Benchmark

The source directory contains a benchmark test suite. To run:

cd .../bench createdb TEST psql TEST < ../_int.sql ./create_test.pl | psql TEST ./bench.pl

The script has numerous options, which are displayed when it is run without any arguments.

Sours: https://postgresql.kr/docs/8.3/intarray.html


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