Column statistics recomputation when collecting column statistics upon updates form 5.0 to 6.0
Details
| Detail name | Value |
|---|---|
| Changelog Number | 5020 |
| Type | Bug |
| Status | Resolved |
| Affected Versions | EXASOL 6.0.0 |
| Fix Versions | EXASOL 6.0.4 |
| Resolution Date | 2017-09-18 |
Backgorund
After an update from EXASOL 5.0 to EXASOL 6.0 the column statistics are recomputed due to format change. This is typically on the fly and as needed, that is when statistics are needed and are not yet recomputed.
Synopsis
The statistics recomputation is not meant to take place when querying the dedicated system table ("$EXA_COLUMN_STATISTICS"), yet it takes place after the update from 5.0 to 6.0 if the statistics have not yet been recomputed.
Effects
Any query on "$EXA_COLUMN_STATISTICS" will recompute the table statistics that have not yet been recomputed after the update to EXASOL 6.0. This may affect overall system performance as much table data may have to be read from disk and processed. In particular, this may have a significant detrimental effect when gathering the debug information described in SOL-520.
Workaround:
Do not query EXASOL internal system tables unless explicitly asked to.
