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Diffstat (limited to 'doc/src/sgml/xoper.sgml')
| -rw-r--r-- | doc/src/sgml/xoper.sgml | 86 |
1 files changed, 43 insertions, 43 deletions
diff --git a/doc/src/sgml/xoper.sgml b/doc/src/sgml/xoper.sgml index 4ee4ac99e20..195acb4714a 100644 --- a/doc/src/sgml/xoper.sgml +++ b/doc/src/sgml/xoper.sgml @@ -186,7 +186,7 @@ SELECT (a + b) AS c FROM test_complex; <para> Providing NEGATOR is very helpful to the query optimizer since it allows expressions like NOT (x = y) to be simplified into - x <> y. This comes up more often than you might think, because + x <> y. This comes up more often than you might think, because NOTs can be inserted as a consequence of other rearrangements. </para> @@ -225,21 +225,21 @@ SELECT (a + b) AS c FROM test_complex; These are the standard restriction estimators: <ProgramListing> eqsel for = - neqsel for <> - intltsel for < or <= - intgtsel for > or >= + neqsel for <> + intltsel for < or <= + intgtsel for > or >= </ProgramListing> It might seem a little odd that these are the categories, but they make sense if you think about it. '=' will typically accept only - a small fraction of the rows in a table; '<>' will typically reject - only a small fraction. '<' will accept a fraction that depends on + a small fraction of the rows in a table; '<>' will typically reject + only a small fraction. '<' will accept a fraction that depends on where the given constant falls in the range of values for that table column (which, it just so happens, is information collected by VACUUM ANALYZE and made available to the selectivity estimator). - '<=' will accept a slightly larger fraction than '<' for the same + '<=' will accept a slightly larger fraction than '<' for the same comparison constant, but they're close enough to not be worth distinguishing, especially since we're not likely to do better than a - rough guess anyhow. Similar remarks apply to '>' and '>='. + rough guess anyhow. Similar remarks apply to '>' and '>='. </para> <para> @@ -249,48 +249,48 @@ SELECT (a + b) AS c FROM test_complex; matching operators (~, ~*, etc) use eqsel on the assumption that they'll usually only match a small fraction of the entries in a table. </para> + </sect2> - <sect2> - <title>JOIN</title> + <sect2> + <title>JOIN</title> - <para> - The JOIN clause, if provided, names a join selectivity - estimation function for the operator (note that this is a function - name, not an operator name). JOIN clauses only make sense for - binary operators that return boolean. The idea behind a join - selectivity estimator is to guess what fraction of the rows in a - pair of tables will satisfy a WHERE-clause condition of the form - <ProgramListing> + <para> + The JOIN clause, if provided, names a join selectivity + estimation function for the operator (note that this is a function + name, not an operator name). JOIN clauses only make sense for + binary operators that return boolean. The idea behind a join + selectivity estimator is to guess what fraction of the rows in a + pair of tables will satisfy a WHERE-clause condition of the form + <ProgramListing> table1.field1 OP table2.field2 - </ProgramListing> - for the current operator. As with the RESTRICT clause, this helps - the optimizer very substantially by letting it figure out which - of several possible join sequences is likely to take the least work. - </para> + </ProgramListing> + for the current operator. As with the RESTRICT clause, this helps + the optimizer very substantially by letting it figure out which + of several possible join sequences is likely to take the least work. + </para> - <para> - As before, this chapter will make no attempt to explain how to write - a join selectivity estimator function, but will just suggest that - you use one of the standard estimators if one is applicable: - <ProgramListing> + <para> + As before, this chapter will make no attempt to explain how to write + a join selectivity estimator function, but will just suggest that + you use one of the standard estimators if one is applicable: + <ProgramListing> eqjoinsel for = - neqjoinsel for <> - intltjoinsel for < or <= - intgtjoinsel for > or >= - </ProgramListing> - </para> + neqjoinsel for <> + intltjoinsel for < or <= + intgtjoinsel for > or >= + </ProgramListing> + </para> + </sect2> - </sect2> + <sect2> + <title>HASHES</title> - <sect2> - <title>HASHES</title> - - <para> - The HASHES clause, if present, tells the system that it is OK to - use the hash join method for a join based on this operator. HASHES - only makes sense for binary operators that return boolean --- and - in practice, the operator had better be equality for some data type. - </para> + <para> + The HASHES clause, if present, tells the system that it is OK to + use the hash join method for a join based on this operator. HASHES + only makes sense for binary operators that return boolean --- and + in practice, the operator had better be equality for some data type. + </para> <para> The assumption underlying hash join is that the join operator can |