sql - Using index on `LIKE :varname || '%'` in firebird -


i have query

select distinct fkdocument pntm_documents_ft_index word 'sometext%'

plan sort ((pntm_documents_ft_index index (ix_pntm_documents_ft_index)))

and works okay.

but when try use concatenated string like, firebird does not use indicies:

select distinct fkdocument pntm_documents_ft_index word 'sometext' || '%'

plan sort ((pntm_documents_ft_index natural))

how force use indicies?

the short answer, ain commented, use starting [with] instead of like if don't need pattern, want prefix search. so:

where word starting 'sometext' -- no %!

or

where word starting :param

as far know firebird like 'sometext%'. use index when available, , don't need escape presence of pattern symbols. downside can't use pattern symbols.

now why firebird doesn't use index when use 

where word :param || '%' -- (or :param) matter

or 

where word 'sometext' || '%'

the first case explained: statement preparation done separately execution. firebird needs take account possibility parameter value starts _ or - worse - %, , can't use index that.

as second case, should possible optimize equivalent of like 'sometext%', firebird considers not plain literal not optimizable. specific example possible decide should optimizable, specific exception (usually 1 doesn't concatenate literals this, of time 1 or more 'black' boxes columns, functions, case statements etc involved).


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