COUNT skips NULL
count(column) counts the rows where the column is not NULL. In the club data, recommended_by is empty for members nobody recommended.
task
How many members were recommended by another member?
Play
lessons
aggregates
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- [ ] a lookup reads few pages
- [ ] b tree meta page
- [ ] boss count a group from the index
- [ ] brin ranges
- [ ] gin pending list
- [ ] index only scans and heap fetches
- [ ] is not distinct from reaches the index
- [ ] physical correlation
- [ ] random inserts leave leaf pages loose
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- [ ] walk the root page
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- [ ] boss one row about a table
- [ ] clear one extended statistics object
- [ ] clear statistics of a missing table
- [ ] column layout in pg attribute
- [ ] count your table s forks
- [ ] extended statistics on virtual generated columns
- [ ] pg stats knows the table oid
- [ ] statistics on a boolean function
- [ ] the index behind a primary key
- [ ] what pg class says about a table
m2-heap-pages
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- [ ] a fresh file after repack
- [ ] a partitioned table needs an index name
- [ ] a read sets all visible without vacuum
- [ ] boss one vacuum cycle
- [ ] dead item memory in vacuum verbose
- [ ] line pointers across delete and vacuum
- [ ] relfrozenxid advances
- [ ] repack concurrently
- [ ] repack using index orders the heap
- [ ] the visibility map after vacuum
- [ ] vacuum full gives the space back
- [ ] which table would autovacuum pick
m5-hot
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planner
- [ ] an impossible predicate
- [ ] coalesce stops at the first not null argument
- [ ] incremental sort under append
- [ ] is not distinct from reaches the index
- [ ] is null on any not null column of the inner side
- [ ] memoize under an anti join
- [ ] read stream counters in explain io
- [ ] read the memoize estimate
- [ ] turn eager aggregation off
ranges
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what-changed-in-19
- [ ] aggregate support clause
- [ ] base64url token
- [ ] boss pg19 table repair
- [ ] check not enforced promote
- [ ] checkpoint options
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- [ ] copy header n
- [ ] copy json header error
- [ ] copy multiline header out
- [ ] copy partitioned roundtrip
- [ ] copy set null
- [ ] distinct from null folding
- [ ] eager aggregate
- [ ] grant granted by
- [ ] group subquery expr
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- [ ] inet gist default
- [ ] is json domain
- [ ] jit off
- [ ] json array empty
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- [ ] oid8 validation
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- [ ] repack concurrently
- [ ] repack index order
- [ ] repack partitioned needs index
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- [ ] scs always on
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- [ ] stemmer dictionaries
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