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- Table indexing

# Table indexing

Once a `Table` has been created, it is possible to create indexes on one or more columns of the table. An index internally sorts the rows of a table based on the index column(s), allowing for element retrieval by column value and improved performance for certain table operations.

Warning

The table indexing engine is new and is not yet considered stable. It is recommended to avoid using this engine in production code for now.

## Creating an index

To create an index on a table, use the `add_index()` method:

    >>> from astropy.table import Table
    >>> t = Table([(2, 3, 2, 1), (8, 7, 6, 5)], names=('a', 'b'))
    >>> t.add_index('a')

The optional argument “unique” may be specified to create an index with uniquely valued elements.

To create a composite index on multiple columns, pass a list of columns instead:

    >>> t.add_index(['a', 'b'])

In particular, the first index created using the `add_index()` method is considered the default index or the “primary key”. To retrieve an index from a table, use the `indices` property:

    >>> t.indices['a']
     a  rows
    --- ----
      1    3
      2    0
      2    2
      3    1
    >>> t.indices['a', 'b']
     a   b  rows
    --- --- ----
      1   5    3
      2   6    2
      2   8    0
      3   7    1

## Row retrieval using indices

Row retrieval can be accomplished using two table properties: `loc` and `iloc`. The `loc` property can be indexed either by column value, range of column values (including the bounds), or a list or ndarray of column values:

    >>> t = Table([(1, 2, 3, 4), (10, 1, 9, 9)], names=('a', 'b'), dtype=['i8', 'i8'])
    >>> t.add_index('a')
    >>> t.loc[2]
    <Row index=1>
      a     b
    int64 int64
    ----- -----
        2     1
    >>> t.loc[[1, 4]]
    <Table length=2>
      a     b
    int64 int64
    ----- -----
        1    10
        4     9
    >>> t.loc[1:3]
    <Table length=3>
      a     b
    int64 int64
    ----- -----
        1    10
        2     1
        3     9
    >>> t.loc[:]
    <Table length=4>
      a     b
    int64 int64
    ----- -----
        1    10
        2     1
        3     9
        4     9

Note that by default, `loc` uses the primary index, which here is column ‘a’. To use a different index, pass the indexed column name before the retrieval data:

    >>> t.add_index('b')
    >>> t.loc['b', 8:10]
    <Table length=3>
      a     b
    int64 int64
    ----- -----
        3     9
        4     9
        1    10

The property `iloc` works similarly, except that the retrieval information must be either an int or a slice, and relates to the sorted order of the index rather than column values. For example:

    >>> t.iloc[0] # smallest row by value 'a'
    <Row index=0>
      a     b
    int64 int64
    ----- -----
        1    10
    >>> t.iloc['b', 1:] # all but smallest value of 'b'
    <Table length=3>
      a     b
    int64 int64
    ----- -----
        3     9
        4     9
        1    10

## Effects on performance

Table operations change somewhat when indices are present, and there are a number of factors to consider when deciding whether the use of indices will improve performance. In general, indexing offers the following advantages:

- Table grouping and sorting based on indexed column(s) become faster
- Retrieving values by index is faster than custom searching

There are certain caveats, however:

- Creating an index requires time and memory
- Table modifications become slower due to automatic index updates
- Slicing a table becomes slower due to index relabeling

See here for an IPython notebook profiling various aspects of table indexing.

## Index modes

The `index_mode()` method allows for some flexibility in the behavior of table indexing by allowing the user to enter a specific indexing mode via a context manager. There are currently three indexing modes: freeze, copy_on_getitem, and discard_on_copy. The freeze mode prevents automatic index updates whenever a column of the index is modified, and all indices refresh themselves after the context ends:

    >>> with t.index_mode('freeze'):
    ...    t['a'][0] = 0
    ...    print(t.indices['a']) # unmodified
     a  rows
    --- ----
      1    0
      2    1
      3    2
      4    3
    >>> print(t.indices['a']) # modified
     a  rows
    --- ----
      0    0
      2    1
      3    2
      4    3

The copy_on_getitem mode forces columns to copy and relabel their indices upon slicing. In the absence of this mode, table slices will preserve indices while column slices will not:

    >>> t['a'][[1, 3]].info.indices
    []
    >>> with t.index_mode('copy_on_getitem'):
    ...    print(t['a'][[1, 3]].info.indices)
    [ a  rows
    --- ----
      2    0
      4    1]

The discard_on_copy mode prevents indices from being copied whenever a column or table is copied:

    >>> t2 = Table(t)
    >>> t2.indices['a']
     a  rows
    --- ----
      0    0
      2    1
      3    2
      4    3
    >>> t2.indices['b']
     b  rows
    --- ----
      1    1
      9    2
      9    3
     10    0
    >>> with t.index_mode('discard_on_copy'):
    ...    t2 = Table(t)
    ...    print(t2.indices)
    []

## Engines

When creating an index via `add_index()`, the keyword argument “engine” may be specified to use a particular indexing engine. The available engines are

- `SortedArray`, a sorted array engine using an underlying sorted Table
- `FastRBT`, a C-based red-black tree engine
- `FastBST`, a C-based binary search tree engine
- `BST`, a Python-based binary search tree engine

Note that FastRBT and FastBST depend on the bintrees dependency; without this dependency, both classes default to `BST`. For a comparison of engine performance, see this IPython notebook. Probably the most important takeaway is that `SortedArray` (the default engine) is usually best, although `FastRBT` may be more appropriate for an index created on an empty column since adding new values is quicker.