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+.\" ========================================================================
+.\"
+.IX Title "funidx 7"
+.TH funidx 7 "April 14, 2011" "version 1.4.5" "SAORD Documentation"
+.SH "NAME"
+Funidx \- Using Indexes to Filter Rows in a Table
+.SH "SYNOPSIS"
+.IX Header "SYNOPSIS"
+This document contains a summary of the user interface for
+filtering rows in binary tables with indexes.
+.SH "DESCRIPTION"
+.IX Header "DESCRIPTION"
+Funtools Table Filtering allows rows in a
+table to be selected based on the values of one or more columns in the
+row. Because the actual filter code is compiled on the fly, it is very
+efficient. However, for very large files (hundreds of Mb or larger),
+evaluating the filter expression on each row can take a long time. Therefore,
+funtools supports index files for columns, which are used automatically during
+filtering to reduce dramatically the number of row evaluations performed.
+The speed increase for indexed filtering can be an order of magnitude or
+more, depending on the size of the file.
+.PP
+The funindex program creates an
+index on one or more columns in a binary table. For example, to create an index
+for the column pi in the file huge.fits, use:
+.PP
+.Vb 1
+\& funindex huge.fits pi
+.Ve
+.PP
+This will create an index named huge_pi.idx.
+.PP
+When a filter expression is initialized for row evaluation, funtools
+looks for an index file for each column in the filter expression. If
+found, and if the file modification date of the index file is later
+than that of the data file, then the index will be used to reduce the
+number of rows that are evaluated in the filter. When
+Spatial Region Filtering is part of the
+expression, the columns associated with the region are checked for index
+files.
+.PP
+If an index file is not available for a given column, then in general,
+all rows must be checked when that column is part of a filter
+expression. This is not true, however, when a non-indexed column is
+part of an \s-1AND\s0 expression. In this case, only the rows that pass the
+other part of the \s-1AND\s0 expression need to be checked. Thus, in some cases,
+filtering speed can increase significantly even if all columns are not
+indexed.
+.PP
+Also note that certain types of filter expression syntax cannot make
+use of indices. For example, calling functions with column names as
+arguments implies that all rows must be checked against the function
+value. Once again, however, if this function is part of an \s-1AND\s0
+expression, then a significant improvement in speed still is possible
+if the other part of the \s-1AND\s0 expression is indexed.
+.PP
+For example, note below the dramatic speedup in searching a 1 Gb
+file using an \s-1AND\s0 filter, even when one of the columns (pha) has no
+index:
+.PP
+.Vb 22
+\& time fundisp \e
+\& huge.fits'[idx_activate=0,idx_debug=1,pha=2348&&cir 4000 4000 1]' \e
+\& "x y pha"
+\& x y pha
+\& ---------- ----------- ----------
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 42.36u 13.07s 6:42.89 13.7%
+.Ve
+.PP
+.Vb 26
+\& time fundisp \e
+\& huge.fits'[idx_activate=1,idx_debug=1,pha=2348&&cir 4000 4000 1]' \e
+\& "x y pha"
+\& x y pha
+\& ---------- ----------- ----------
+\& idxeq: [INDEF]
+\& idxand sort: x[ROW 8037025:8070128] y[ROW 5757665:5792352]
+\& idxand(1): INDEF [IDX_OR_SORT]
+\& idxall(1): [IDX_OR_SORT]
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 3999.48 4000.47 2348
+\& 1.55u 0.37s 1:19.80 2.4%
+.Ve
+.PP
+When all columns are indexed, the increase in speed can be even more dramatic:
+.PP
+.Vb 22
+\& time fundisp \e
+\& huge.fits'[idx_activate=0,idx_debug=1,pi=770&&cir 4000 4000 1]' \e
+\& "x y pi"
+\& x y pi
+\& ---------- ----------- ----------
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 42.60u 12.63s 7:28.63 12.3%
+.Ve
+.PP
+.Vb 27
+\& time fundisp \e
+\& huge.fits'[idx_activate=1,idx_debug=1,pi=770&&cir 4000 4000 1]' \e
+\& "x y pi"
+\& x y pi
+\& ---------- ----------- ----------
+\& idxeq: pi start=9473025,stop=9492240 => pi[ROW 9473025:9492240]
+\& idxand sort: x[ROW 8037025:8070128] y[ROW 5757665:5792352]
+\& idxor sort/merge: pi[ROW 9473025:9492240] [IDX_OR_SORT]
+\& idxmerge(5): [IDX_OR_SORT] pi[ROW]
+\& idxall(1): [IDX_OR_SORT]
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 3999.48 4000.47 770
+\& 1.67u 0.30s 0:24.76 7.9%
+.Ve
+.PP
+The miracle of indexed filtering (and indeed, of any indexing) is the
+speed of the binary search on the index, which is of order log2(n)
+instead of n. (The funtools binary search method is taken from
+http://www.tbray.org/ongoing/When/200x/2003/03/22/Binary, to whom
+grateful acknowledgement is made.) This means that the larger the
+file, the better the performance. Conversely, it also means that for
+small files, using an index (and the overhead involved) can slow
+filtering down somewhat. Our tests indicate that on a file containing
+a few tens of thousands of rows, indexed filtering can be 10 to 20
+percent slower than non-indexed filtering. Of course, your mileage
+will vary with conditions (disk access speed, amount of available
+memory, process load, etc.)
+.PP
+Any problem encountered during index processing will result in
+indexing being turned off, and replaced by filtering all rows. You can turn
+filtering off manually by setting the idx_activate variable to 0 (in a filter
+expression) or the \s-1FILTER_IDX_ACTIVATE\s0 environment variable to 0 (in the global
+environment). Debugging output showing how the indexes are being processed can
+be displayed to stderr by setting the idx_debug variable to 1 (in a filter
+expression) or the \s-1FILTER_IDX_DEBUG\s0 environment variable to 1 (in the global
+environment).
+.PP
+Currently, indexed filtering only works with \s-1FITS\s0 binary tables and raw
+event files. It does not work with text files. This restriction might be
+removed in a future release.
+.SH "SEE ALSO"
+.IX Header "SEE ALSO"
+See funtools(7) for a list of Funtools help pages
generated by cgit v0.12 at 2024-08-29 00:36:43 (GMT)