(Unless plain x is also involved in the query,) INCLUDE (x) only serves as awkward hint for the query planner, while only f(x) is actually used.Īnswer 2: "Wide" columns are big columns, columns that occupy a lot of storage "on disk" (often not a "disk" nowadays) or in RAM. Worked around by adding x as an included column, for exampleĬREATE INDEX tab_f_x ON tab (f(x)) INCLUDE (x) If an index-only scan seems sufficiently worthwhile, this can be Postgres is currently (Postgres 15) not smart enough to chose an index-only scan unless the involved column itself is included in the index. Plus, there is a case with expression indexes. The benefit is biggest for large cardinalities (multiple index levels), and little duplication (suffix truncation can't make up for not moving a payload column to the INCLUDE part). But the upper levels are the ones that have to be read all the time. Now, B-tree indexes are only few levels deep. However, explicitlyĭefining payload columns as non-key columns reliably keeps the tuples Indexes without an INCLUDE clause often avoid storing columns thatĪre effectively payload in the upper levels. When the remaining prefix of key column(s) happens to be sufficient toĭescribe tuples on the lowest B-Tree level. The truncation process also removes one or more trailing key column(s) As payload columns, they are never used to guide index scans. Suffix truncation always removes non-key columns from upper B-Tree levels. But there are still minor benefits for other cases. And in some cases not even then.Īnswer 1: The INCLUDE feature is predominantly useful for the two excluded cases: uniqueness, or not allowed in the index otherwise. Rule of thumb 2: INCLUDE columns only ever make sense if you actually get index-only scans from them. Rule of thumb 1: If you never use an index column for filtering or sorting (or joining, or to enforce uniqueness), you might as well move it to the INCLUDE clause. In any case, non-key columns duplicate data from the index's table and Then how does the below statement not hold for INCLUDE(y). If INCLUDE supports index only scans then y will also have to be stored in index. Question 3: Can someone explain the below statement in context of INCLUDE(y). Question 2: Can someone explain with an example what wide columns mean? InĪny case, non-key columns duplicate data from the index's table andīloat the size of the index, thus potentially slowing searches. Maximum size allowed for the index type, data insertion will fail. It's wise to be conservative about adding non-key payload columns toĪn index, especially wide columns. Question 1: If the data type of y can be added in index and there is no uniqueness requirement then is there any advantage of using CREATE INDEX tab_x_y ON tab(x) INCLUDE (y) over CREATE INDEX tab_x_y ON tab(x, y) for queries like SELECT y FROM tab WHERE x = 'key' ? UNIQUE and PRIMARY KEY constraints, providing alternative syntax for (An INCLUDE clause can also be written in The uniqueness condition applies to just column x, not to theĬombination of x and y. Stored in the index and is not interpreted by the index machinery.Īlso, if the index is a unique index, that isĬREATE UNIQUE INDEX tab_x_y ON tab(x) INCLUDE (y) Have to be of a data type that the index can handle it's merely Obtained from the index without visiting the heap.īecause column y is not part of the index's search key, it does not However, an index defined asĬREATE INDEX tab_x_y ON tab(x) INCLUDE (y) Ĭould handle these queries as index-only scans, because y can be The traditional approach to speeding up such queries would be toĬreate an index on x only. Postgres docs state the following about Index-Only Scans and Covering-Indexes:
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