The strategy of GIN index scan is driven by opclass-specific extract_query method. This method that needed search mode is GIN_SEARCH_MODE_ALL. This mode means that matching tuple may contain none of extracted entries. Simple example is '!term' tsquery, which doesn't need any term to exist in matching tsvector.
In order to handle such scan key GIN calculates virtual entry, which contains all TIDs of all entries of attribute. In fact this is full scan of index attribute. And typically this is very slow, but allows to handle some queries correctly in GIN. However, current algorithm calculate such virtual entry for each GIN_SEARCH_MODE_ALL scan key even if they are multiple for the same attribute. This is clearly not optimal.
This commit improves the situation by introduction of "exclude only" scan keys. Such scan keys are not capable to return set of matching TIDs. Instead, they are capable only to filter TIDs produced by normal scan keys. Therefore, each attribute should contain at least one normal scan key, while rest of them may be "exclude only" if search mode is GIN_SEARCH_MODE_ALL.
The same optimization might be applied to the whole scan, not per-attribute. But that leads to NULL values elimination problem. There is trade-off between multiple possible ways to do this. We probably want to do this later using some cost-based decision algorithm.
Discussion: https://postgr.es/m/CAOBaU_YGP5-BEt5Cc0%3DzMve92vocPzD%2BXiZgiZs1kjY0cj%3DXBg%40mail.gmail.com Author: Nikita Glukhov, Alexander Korotkov, Tom Lane, Julien Rouhaud
4b754d6c16 Avoid full scan of GIN indexes when possible
contrib/pg_trgm/expected/pg_trgm.out | 101 ++++++++++++++++++++++++++
contrib/pg_trgm/sql/pg_trgm.sql | 27 +++++++
src/backend/access/gin/ginget.c | 84 ++++++++++++++++------
src/backend/access/gin/ginscan.c | 127 +++++++++++++++++++-------------
src/backend/utils/adt/selfuncs.c | 37 ++++++++--
src/include/access/gin_private.h | 14 ++++
src/test/regress/expected/gin.out | 131 +++++++++++++++++++++++++++++++++-
src/test/regress/expected/tsearch.out | 35 +++++++++
src/test/regress/sql/gin.sql | 91 ++++++++++++++++++++++-
src/test/regress/sql/tsearch.sql | 9 +++
10 files changed, 579 insertions(+), 77 deletions(-)