PostGIS Raster : Extending PostgreSQL for The Support of ... - CoDE

Figure 6.8: The color legend interface.
QGIS retrieves the temporal raster table from PostgreSQL using a connection string and a SQL
command. The connection string is used to establish a connection to PostgreSQL. Its structure is
’PG: dbname=%s host=%s user=%s password=%s port=%s’. PostgreSQL then executes the SQL
command and outputs temporal raster table list as a result.
6.3.1 Time Travel
Loading Data
To show the revolution in time of a set of temporal rasters, first of all, data need to be loaded
from PostgreSQL into QGIS. There exist two approaches for loading temporal raster (or map). The
first one attempts to load each map, one after another from PostgreSQL database into QGIS. When
loading a map, there is a change from QGIS cursor to PostgreSQL cursor. That is because when QGIS
establishes a connection to PostgreSQL, PostgreSQL will execute a request to retrieve a temporal raster
row. When the map is loaded, necessary PyQt instructions like updating current time, progress slider
bar etc. are need to be executed to show the evolution. In turn, a cursor change from PostgreSQL to
QGIS occurs again before these instructions take place, as PyQt instructions are managed by QGIS
cursor. These steps are exactly what will happen to the next loaded map and so on until the final
loaded map. This approach sounds cumbersome and involves a lot of execution time due to cursor
changes.
Figure 6.9: The loading map sequential approach.
In the second approach, all maps are loaded into QGIS memory once and for all. Hence, only two
cursor changes are needed (one from QGIS to PostgreSQL and another from PostgreSQL to QGIS).
Then, the visibility of each map is simply handled by turning on or off the check box corresponding to
this map from the layers panel as described in the previous section. This approach reduces enormously
execution time. Another advantage results from this is that all maps can be organized in an order
(descending or ascending) by modifying the SQL command. The ordering can not happen in the first
approach as each map is individually loaded.
Pseudo code of function iniLayers that loads all layers into QGIS using the second approach:
def iniLayers(self):
Retrieve a list of temporal raster IDs to rids
Change QGIS cursor to PostgreSQL cursor
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