diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/indices.sgml | 24 |
1 files changed, 18 insertions, 6 deletions
diff --git a/doc/src/sgml/indices.sgml b/doc/src/sgml/indices.sgml index 9943e8ecd4c..3493f482b86 100644 --- a/doc/src/sgml/indices.sgml +++ b/doc/src/sgml/indices.sgml @@ -504,8 +504,9 @@ CREATE INDEX test2_mm_idx ON test2 (major, minor); <para> By default, B-tree indexes store their entries in ascending order - with nulls last. This means that a forward scan of an index on - column <literal>x</literal> produces output satisfying <literal>ORDER BY x</literal> + with nulls last (table TID is treated as a tiebreaker column among + otherwise equal entries). This means that a forward scan of an + index on column <literal>x</literal> produces output satisfying <literal>ORDER BY x</literal> (or more verbosely, <literal>ORDER BY x ASC NULLS LAST</literal>). The index can also be scanned backward, producing output satisfying <literal>ORDER BY x DESC</literal> @@ -1162,10 +1163,21 @@ CREATE INDEX tab_x_y ON tab(x, y); the extra columns are trailing columns; making them be leading columns is unwise for the reasons explained in <xref linkend="indexes-multicolumn"/>. However, this method doesn't support the case where you want the index to - enforce uniqueness on the key column(s). Also, explicitly marking - non-searchable columns as <literal>INCLUDE</literal> columns makes the - index slightly smaller, because such columns need not be stored in upper - tree levels. + enforce uniqueness on the key column(s). + </para> + + <para> + <firstterm>Suffix truncation</firstterm> always removes non-key + columns from upper B-Tree levels. As payload columns, they are + never used to guide index scans. The truncation process also + removes one or more trailing key column(s) when the remaining + prefix of key column(s) happens to be sufficient to describe tuples + on the lowest B-Tree level. In practice, covering indexes without + an <literal>INCLUDE</literal> clause often avoid storing columns + that are effectively payload in the upper levels. However, + explicitly defining payload columns as non-key columns + <emphasis>reliably</emphasis> keeps the tuples in upper levels + small. </para> <para> |