Struct odbc_api::buffers::ColumnarBuffer

pub struct ColumnarBuffer<C> { /* private fields */ }

A columnar buffer intended to be bound with crate::Cursor::bind_buffer in order to obtain results from a cursor.

This buffer is designed to be versatile. It supports a wide variety of usage scenarios. It is efficient in retrieving data, but expensive to allocate, as columns are allocated separately. This is required in order to efficiently allow for rebinding columns, if this buffer is used to provide array input parameters those maximum size is not known in advance.

Most applications should find the overhead negligible, especially if instances are reused.

Implementations

impl<C: ColumnBuffer> ColumnarBuffer<C>

pub fn new(columns: Vec<(u16, C)>) -> Self

Create a new instance from columns with unique indicies. Capacity of the buffer will be the minimum capacity of the columns.

pub unsafe fn new_unchecked(capacity: usize, columns: Vec<(u16, C)>) -> Self

Safety

• Indices must be unique
• Columns all must have enough capacity.

pub fn num_rows(&self) -> usize

Number of valid rows in the buffer.

pub fn num_cols(&self) -> usize

Return the number of columns in the row set.

pub fn column(&self, buffer_index: usize) -> <C as ColumnProjections<'_>>::View

Use this method to gain read access to the actual column data.

Parameters

• buffer_index: Please note that the buffer index is not identical to the ODBC column index. For once it is zero based. It also indexes the buffer bound, and not the columns of the output result set. This is important, because not every column needs to be bound. Some columns may simply be ignored. That being said, if every column of the output is bound in the buffer, in the same order in which they are enumerated in the result set, the relationship between column index and buffer index is buffer_index = column_index - 1.

pub fn column_mut(
    &mut self,
    buffer_index: usize
) -> <C as ColumnProjections<'_>>::ViewMut

Use this method to gain write access to the actual column data.

Parameters

• buffer_index: Please note that the buffer index is not identical to the ODBC column index. For once it is zero based. It also indexes the buffer bound, and not the columns of the output result set. This is important, because not every column needs to be bound. Some columns may simply be ignored. That being said, if every column of the output is bound in the buffer, in the same order in which they are enumerated in the result set, the relationship between column index and buffer index is buffer_index = column_index - 1.

Example

This method is intend to be called if using ColumnarBuffer for column wise bulk inserts.

use odbc_api::{
    Connection, Error, IntoParameter,
    buffers::{BufferDescription, BufferKind, AnyColumnViewMut, buffer_from_description}
};

fn insert_birth_years(conn: &Connection, names: &[&str], years: &[i16])
    -> Result<(), Error>
{

    // All columns must have equal length.
    assert_eq!(names.len(), years.len());

    // Create a columnar buffer which fits the input parameters.
    let buffer_description = [
        BufferDescription {
            kind: BufferKind::Text { max_str_len: 255 },
            nullable: false,
        },
        BufferDescription {
            kind: BufferKind::I16,
            nullable: false,
        },
    ];
    let mut buffer = buffer_from_description(
        names.len(),
        buffer_description.iter().copied()
    );

    // Fill the buffer with values column by column
    match buffer.column_mut(0) {
        AnyColumnViewMut::Text(mut col) => {
            col.write(names.iter().map(|s| Some(s.as_bytes())))
        }
        _ => panic!("We know the name column to hold text.")
    }

    match buffer.column_mut(1) {
        AnyColumnViewMut::I16(mut col) => {
            col.copy_from_slice(years)
        }
        _ => panic!("We know the year column to hold i16.")
    }

    conn.execute(
        "INSERT INTO Birthdays (name, year) VALUES (?, ?)",
        &buffer
    )?;
    Ok(())
}

pub fn set_num_rows(&mut self, num_rows: usize)

Set number of valid rows in the buffer. May not be larger than the batch size. If the specified number should be larger than the number of valid rows currently held by the buffer additional rows with the default value are going to be created.

pub fn clear(&mut self)

Sets the number of rows in the buffer to zero.

impl ColumnarBuffer<TextColumn<u8>>

pub fn for_cursor(
    batch_size: usize,
    cursor: &impl ResultSetMetadata,
    max_str_len: Option<usize>
) -> Result<TextRowSet, Error>

The resulting text buffer is not in any way tied to the cursor, other than that its buffer sizes a tailor fitted to result set the cursor is iterating over.

Parameters

• batch_size: The maximum number of rows the buffer is able to hold.
• cursor: Used to query the display size for each column of the row set. For character data the length in characters is multiplied by 4 in order to have enough space for 4 byte utf-8 characters. This is a pessimization for some data sources (e.g. SQLite 3) which do interpret the size of a VARCHAR(5) column as 5 bytes rather than 5 characters.
• max_str_limit: Some queries make it hard to estimate a sensible upper bound and sometimes drivers are just not that good at it. This argument allows you to specify an upper bound for the length of character data.

pub fn from_max_str_lens(
    row_capacity: usize,
    max_str_lengths: impl Iterator<Item = usize>
) -> Self

Creates a text buffer large enough to hold batch_size rows with one column for each item max_str_lengths of respective size.

pub fn at(&self, buffer_index: usize, row_index: usize) -> Option<&[u8]>

Access the element at the specified position in the row set.

pub fn at_as_str(
    &self,
    col_index: usize,
    row_index: usize
) -> Result<Option<&str>, Utf8Error>

Access the element at the specified position in the row set.

pub fn indicator_at(&self, buf_index: usize, row_index: usize) -> Indicator

Indicator value at the specified position. Useful to detect truncation of data.

Example

use odbc_api::buffers::{Indicator, TextRowSet};

fn is_truncated(buffer: &TextRowSet, col_index: usize, row_index: usize) -> bool {
    match buffer.indicator_at(col_index, row_index) {
        // There is no value, therefore there is no value not fitting in the column buffer.
        Indicator::Null => false,
        // The value did not fit into the column buffer, we do not even know, by how much.
        Indicator::NoTotal => true,
        Indicator::Length(total_length) => {
            // If the maximum string length is shorter than the values total length, the
            // has been truncated to fit into the buffer.
            buffer.max_len(col_index) < total_length
        }
    }
}

pub fn max_len(&self, buf_index: usize) -> usize

Maximum length in bytes of elements in a column.

pub fn append<'a>(&mut self, row: impl Iterator<Item = Option<&'a [u8]>>)

Takes one element from the iterator for each internal column buffer and appends it to the end of the buffer. Should the buffer be not large enough to hold the element, it will be reallocated with 1.2 times its size.

This method panics if it is tried to insert elements beyond batch size. It will also panic if row does not contain at least one item for each internal column buffer.

Trait Implementations

impl<C> ParameterRefCollection for &ColumnarBuffer<C> where
    C: ColumnBuffer + HasDataType, 

fn parameter_set_size(&self) -> usize

Number of values per parameter in the collection. This can be different from the maximum batch size a buffer may be able to hold. Returning 0 will cause the the query not to be executed.

unsafe fn bind_parameters_to(
    &mut self,
    stmt: &mut impl Statement
) -> Result<(), Error>

Safety

impl<C> RowSetBuffer for ColumnarBuffer<C> where
    C: ColumnBuffer, 

fn bind_type(&self) -> usize

Declares the bind type of the Row set buffer. 0 Means a columnar binding is used. Any non zero number is interpreted as the size of a single row in a row wise binding style.

fn row_array_size(&self) -> usize

The batch size for bulk cursors, if retrieving many rows at once.

fn mut_num_fetch_rows(&mut self) -> &mut usize

Mutable reference to the number of fetched rows.

unsafe fn bind_to_cursor(
    &mut self,
    cursor: &mut impl Cursor
) -> Result<(), Error>

Binds the buffer either column or row wise to the cursor.