Query dict

This document describes how to generate a query dict for use with a specdb database.

Basics

Aside from coordinate searches, queries of tables in a specdb database are managed by a query dict. The dict keys refer to the table columns and the dict values are used to perform the query.

The following table summarizes how different types of values in a query dict are treated:

Value type	Example	Interpretation
tuple	(1.,2.)	Interpreted as the min/max range for a query
int	1	Interpreted as a single value to match against
float	1.0	Interpreted as a single value to match against
str	‘2’	Interpreted as a single value to match against
list	[2,32,256]	Interpreted as a list of values to match against
		Data type should match that of the table column

For list entries, the match is successful if the Table values matches any of the items in the list.

Examples for Catalog Queries

While most queries of the database catalog will be based on coordinates, one can further restrict the query with a query dict.

Here are some simple examples for querying the Source Catalog:

qdict = {'zem': (1.,2.), 'STYPE': 'QSO'}

This will restrict to sources with redshift 1<=z<=2 and source type ‘QSO’. Here is another:

qdict = {'zem': (1.,2.), 'flag_group': 8}

Same cut on redshift and now restricting to sources with the bitwise flag_group=8.

Examples for Meta Queries

In general, one uses meta queries for items related to the spectra. Here is one to query on high resolution:

qdict = {'R': (20000.,1e9)}

This will limit to echelle resolution.

Bitwise Flags

Extra handling is required to query bitwise flags in a bitwise fashion. The current approach is to append ‘-BITWISE’ to the keyword to indicate that the input value(s) is/are to be matched in a bitwise manner. In addition, one appends ‘-AND’ to require all of the bits are on or ‘-OR’ to require only one. Here is an example:

qdict = {'zem': (1.,2.), 'flag_group-BITWISE-OR': [2,4,32]}

This will match against the flag_group column with bitwise logic. That is, Table rows with any of the input values turned on will match.

Note that the input values are the full values, i.e. 2**5=32 or 2**9=512, not 5 or 9.