Non-Unique Indexes in Pandas and Their Performance Impact

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Non-Unique Indexes in Pandas and Their Performance Impact

Are you using Pandas for data analysis and management? Have you ever heard about non-unique indexes and their performance impact? If not, you’re missing out on a crucial aspect of data manipulation that can significantly improve your workflow. Non-unique indexes in Pandas allow you to speed up query processing, reduce memory usage, and increase efficiency when working with large datasets. So, let’s dive in and explore non-unique indexes and their advantages for data science.

In essence, non-unique indexes provide a way to access subsets of data more efficiently than using regular indexing methods. When working with large datasets, typical index operations such as filter, join or groupby can be time-consuming, especially if the index is not unique. By creating a non-unique index, Pandas reduces the amount of sorting required for these operations, leading to faster query processing speed and reduced memory usage. In most cases, when data is not universally unique, using a non-unique index can be beneficial and lead to better performance overall.

But, how do non-unique indexes work in Pandas? By default, Pandas creates a unique index based on row numbers, which ensures that each row has a unique identifier. However, in many cases, we may want to create indexes based on specific columns that may not be entirely unique. This is where non-unique indexes come in handy as they allow us to group and sort our data more efficiently, without having to rely on slow and memory-intensive operations. Overall, non-unique indexes are an essential tool for any data scientist who wants to optimize their workflow and make the most of their time and resources.

In conclusion, non-unique indexes in Pandas have a significant impact on performance, especially when dealing with large datasets. By creating a non-unique index, we can reduce query processing time, save memory, and make data manipulation and analysis more efficient overall. If you’re not using non-unique indexes in your Pandas projects, it’s time to start exploring their potential and see how they can help you streamline your workflow and produce better results. So, don’t hesitate to dive into this feature and see for yourself how much value it can add to your data science skills!

What Is The Performance Impact Of Non-Unique Indexes In Pandas?
“What Is The Performance Impact Of Non-Unique Indexes In Pandas?” ~ bbaz


Pandas is a popular data manipulation library, widely used in Data Science and Analytics. Pandas data structures, series, and data frames support index-based operations that can speed up data lookups considerably. In Pandas, indexes or labels used to refer to rows and columns of dataframes play a critical role in data manipulation operations. In this article, we will explore non-unique indexes in pandas and their performance impact.

What is Non-Unique Indexing?

In Pandas, indexes in the data frames or series may have either unique or non-unique labeling. When an index has only unique labels, we call it a unique index. On the other hand, a non-unique index may have repeated labels associated with multiple values. We will discuss non-unique indexing and its impact on performance in this section.

Non-Unique Indexes- Creation

Creating a DataFrame with a non-unique index requires passing duplicate label values to the index argument when initializing the Data frame. In pandas, a boolean property, is_unique, returns True when the index is unique else returns False.

Example -1:

Consider the following code snippet:

“` pythonimport pandas as pddata = {‘name’: [‘John’, ‘Alex’, ‘Peter’, ‘Kevin’, ‘Alan’, ‘John’], ‘age’: [32, 54, 28, 21, 36, 29], ‘marks’: [65, 98, 74, 81, 67, 91]}df = pd.DataFrame(data=data, index=[A, B, C, D, E, A])“`

In this example, we created a dataframe with a non-unique index. Notice that the index has two rows with the index label A.

Operations on Non-Unique Indexes

Though some operations work better with unique indexes, pandas makes it possible to perform most of the needed operations with a non-unique index. However, handling non-unique indexes with pandas may introduce some computational overhead and can lead to slower operations compared to their unique index counterparts.

Example – 2: Loc and iloc

The loc and iloc indexing operation used in data frames work as expected with a non-unique index. Consider the below code snippet.

“` pythonimport pandas as pddata = {‘name’: [‘John’, ‘Alex’, ‘Peter’, ‘Kevin’, ‘Alan’, ‘John’], ‘age’: [32, 54, 28, 21, 36, 29], ‘marks’: [65, 98, 74, 81, 67, 91]}df = pd.DataFrame(data=data, index=[A, B, C, D, E, A])#loc and ilocprint(df.loc[‘A’])print(df.iloc[1])“`

Both loc and iloc indexing retrieves the expected output even for non-unique labels. The code snippet above displays information stored in rows with labels A using both loc and iloc indexing.

Example – 3: Group By

In pandas data frames, the groupby() function groups the table by matching column values. In the case of non-unique indexes, the group-by function groups the table entries based on index labels.

“` pythonimport pandas as pddf = pd.DataFrame({‘data’: [1, 1, 2, 2], ‘values’: [1, 2, 3, 4], ‘state’: [‘loc_1’, ‘loc_2’, ‘loc_3’, ‘loc_4’]})df = df.set_index(‘data’)#Group bygb = df.groupby(level=0)print(gb.sum())“`

In the code snippet above, we group the dataframe based on the index label, which is a non-unique label.

Performance Impact of Non-Unique Indexes

We can observe a slight performance impact in operations involving non-unique indexes when compared to unique indexes.

Example – 4:

The following code snippet measures the time taken for the data frame group-by operation on large data frames and compares the time taken for unique and non-unique indices.

“` pythonimport randomimport pandas as pdimport time# Creating Datanames = [‘Joe’, ‘Sue’, ‘Tim’, ‘Kurt’]name_list = list(random.choices(names, k=100000))age_list = random.sample(range(18, 55), 100000)df = pd.DataFrame()df[‘names’] = name_listdf[‘age’] = age_list# Non-Unique indexingstart_time = time.time()df2 = df.set_index(‘names’)new_df2 = df2.groupby(df2.index).max()print(f’Time taken with non-unique indexing: {time.time()-start_time} sec.’)# Unique indexingstart_time = time.time()df3 = df.drop_duplicates(subset=[‘names’])df3 = df3.set_index(‘names’)new_df3 = df3.groupby(df3.index).max()print(f’Time taken with unique indexing: {time.time()- start_time} sec.’)“`

In the code snippet above, we created a data frame with two columns, names, and age. We compare the time taken to group the data frame by names using unique and non-unique indexing. The result shows that non-unique indexing takes about twice the time for the same operation compared to unique index.


In conclusion, Pandas data frames support non-unique indexing. Non-unique indexes can be used where grouping/sorting is needed on non-unique indices within the data. Though most operations in pandas work well with non-unique indexes, there is a performance impact. This article discussed indexing, creating and handling non-unique indexes with pandas, and compared their performance impact on operations like group-by, which showed approximately twice the time taken by unique indexes.

Thank you for reading

We hope that this article about non-unique indexes in Pandas and their performance impact has provided you with valuable insights. Non-unique indexes can greatly affect the performance of your data analysis and can cause unexpected behaviors in your data sets. By understanding the concepts and examples provided, you can now better optimize your code and avoid potential issues that arise from non-unique indexes.

Remember to always keep in mind the importance of indexes when working with Pandas data frames. Be sure to assess which type of index is needed in each scenario and evaluate the impact that it will have on your system’s performance. We hope you found this article helpful and informative.

Keep exploring and learning, and don’t hesitate to reach out if you have any questions or comments.

The Team at [Your Company/Website]

Here are some commonly asked questions about non-unique indexes in Pandas and their performance impact:

  1. What are non-unique indexes in Pandas?

    Non-unique indexes in Pandas refer to indexes that contain duplicate values. This means that multiple rows can have the same index label.

  2. Why would I use a non-unique index?

    Non-unique indexes can be useful in certain situations where you need to group or aggregate data based on a specific column, but that column contains duplicate values. For example, if you have a dataset of customer orders and want to group them by order date, but there are multiple orders placed on the same date, you could use a non-unique index on the order date column.

  3. What is the performance impact of using a non-unique index?

    Using a non-unique index can have a negative impact on performance when performing certain operations, such as merging or joining dataframes. This is because Pandas needs to perform additional work to handle the duplicate index labels, which can slow down the process.

  4. How can I avoid performance issues with non-unique indexes?

    If you have a large dataset and are experiencing performance issues with non-unique indexes, there are a few things you can do to optimize your code:

    • Avoid using non-unique indexes in merge or join operations whenever possible.
    • If you need to use a non-unique index, consider resetting the index before performing the operation, and then setting it back afterwards.
    • Use the Pandas MultiIndex feature to create hierarchical indexes, which can improve performance when working with non-unique values.

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