ST_ColorTable-PolarDB(PolarDB)-阿里云帮助中心

Returns the color table of a raster band as a JSON string, or null if the band has no color table.

Syntax

text ST_ColorTable(raster raster_obj, integer band);

Parameters

Parameter	Description
`raster_obj`	The raster object.
`band`	The band index, starting from `0`.

Description

ST_ColorTable retrieves the color lookup table attached to a specific band of a raster object. If the band does not have a color table, the function returns null.

The returned JSON contains a compsCount field and an entries array. Each entry maps a pixel value to its color components:

Field	Description
`compsCount`	Number of color components: `3` for three-component color, `4` for four-component color.
`value`	The pixel value in the color table.
`c1`	First color component value.
`c2`	Second color component value.
`c3`	Third color component value.
`c4`	Fourth color component value. Present only when `compsCount` is `4`.

Three-component format:

{
  "compsCount": 3,
  "entries": [
    {"value": 0, "c1": 0, "c2": 0, "c3": 0},
    {"value": 1, "c1": 0, "c2": 0, "c3": 85},
    {"value": 2, "c1": 0, "c2": 0, "c3": 170}
  ]
}

Four-component format:

{
  "compsCount": 4,
  "entries": [
    {"value": 0, "c1": 0, "c2": 0, "c3": 0,   "c4": 255},
    {"value": 1, "c1": 0, "c2": 0, "c3": 85,  "c4": 255},
    {"value": 2, "c1": 0, "c2": 0, "c3": 170, "c4": 255}
  ]
}

Examples

Example 1: Query a band with a color table

SELECT ST_ColorTable(raster_obj, 0)
FROM raster_table
WHERE id = 1;

Result:

{
  "compsCount": 3,
  "entries": [
    {"value": 0, "c1": 0, "c2": 0, "c3": 0},
    {"value": 1, "c1": 0, "c2": 0, "c3": 85},
    {"value": 2, "c1": 0, "c2": 0, "c3": 170}
  ]
}

Example 2: Filter rows that have a color table on band 0

Use IS NOT NULL to skip bands that do not have a color table:

SELECT id, ST_ColorTable(raster_obj, 0)
FROM raster_table
WHERE ST_ColorTable(raster_obj, 0) IS NOT NULL;