6.7. Query Planning¶
Query planning is the process of translating a GeoTools
Query into scans and filters for a
particular back-end. Query planning in GeoMesa consists of several steps:
- The CQL filter (if any) is re-written and optimized for fast evaluation
- The CQL filter is split apart, based on the available indices
- One of the available indices is selected to execute the query
- A logical query plan is created by the core GeoMesa indexing code
- A physical query plan is created for the particular back-end database
6.7.1. Filter Decomposition¶
A logical query plan in GeoMesa generally consists of a ‘primary’ CQL filter, which is used to determine scan ranges, and a ‘secondary’ CQL filter which is applied to matching rows. For example, the Z2 index will handle any spatial predicates as scan ranges, and any additional filters will be applied afterwards.
During step two of query planning, the full filter is decomposed and examined with an eye towards the available indices. For each index, a primary and a secondary filter will be determined (if any).
For example, consider the filter
BBOX(geom,0,0,10,10) AND dtg DURING 2017-01-01T00:00:00.000Z/2017-01-02T00:00:00.000Z AND name = 'alice'.
This filter can be decomposed several ways - for the Z2 spatial index, the primary filter is the
the Z3 spatio-temporal index, the primary filter is the
BBOX plus the
DURING, and for the attribute index
(assuming ‘name’ is indexed) the primary filter is
name = 'alice'.
6.7.2. Index Selection¶
Since skipping rows entirely is much faster than reading and filtering them, the best query plan will generally be the one that scans the fewest rows. In other words, the best plan is the one that has the most selective primary filter. GeoMesa has two methods for determining the best index - cost-based, or heuristic-based. Cached statistics are used to estimate the cost when available; however the method used can be configured per-query. See Query Index and Query Planning Type for more information.
126.96.36.199. Cost-Based Strategy¶
Cached statistics, and thus cost-based query planning, are currently only implemented for the Accumulo data store
GeoMesa will collect stats during ingestion, and store them for use in query planning. The stats collected are:
- Total count
- Min/max (bounds) for the default geometry, default date and any indexed attributes
- Histograms for the default geometry, default date and any indexed attributes
- Frequencies for any indexed attributes, split up by week
- Top-k for any indexed attributes
- Z3 histogram based on the default geometry and default date (if both present)
These stats are used to estimate the number of features matching a given primary filter. The primary filter that matches the fewest features will be selected.
188.8.131.52. Heuristic Strategy¶
Heuristics can be used for query planning based solely on the query filter. The priorities are:
- Feature ID predicates using the ID index
- High-cardinality attribute predicates using the attribute index
- Attribute equality predicates using the attribute index
- Spatio-temporal predicates using the Z3/XZ3 index
- Attribute range predicates using the attribute index
- Spatial predicates using the Z2/XZ2 index
- Temporal predicates using the Z3/XZ3 index
- Low-cardinality attribute predicates using the attribute index
In addition, Accumulo data stores using ‘join’ attribute indices will de-prioritize any predicates that require a join, based on the query properties/transform.
If multiple attribute predicates are tied for highest priority, then there is no guarantee about which one will be selected from that group.
184.108.40.206. Custom Strategies¶
It is possible to use custom strategy implementations by specifying the class name with the system property
geomesa.strategy.decider. The class must implement
220.127.116.11. Cardinality Hints¶
Attributes that are know to have many distinct values, i.e. a high cardinality, are likely to filter out many false positives through the index structure, and thus a query against the attribute index will touch relatively few records. Conversely, in the worst case, a Boolean attribute (for example), with only two distinct values, would likely require scanning half of the entire data set.
Cardinality hints may be used to influence the query planner when considering attribute indices. If an attribute is marked as having a high cardinality, the attribute index will be prioritized. Conversely, if an attribute is marked with low cardinality, the attribute index will be de-prioritized. For details on setting cardinality, see Configuring Attribute Cardinality.
6.8. Explaining Query Plans¶
GeoMesa will automatically log explain plans during query execution. This can be useful when debugging query issues, and can inform decisions to speed up execution time, such as when to add additional indices or when query hints may be helpful.
In order to show explain logging, configure your logging system to set
trace level. For example, in log4j use:
Instead of passively logging, you can also generate explain logging without executing a query. Given a GeoMesa data store and a query, use the following method:
import org.locationtech.geomesa.index.utils.ExplainString dataStore.getQueryPlan(query, explainer = new ExplainPrintln)
ExplainPrintln will write to
System.out. Alternatively, you can use
ExplainLogging to redirect the output elsewhere.
Using the binary distribution, you can print out an explain plan using the
explain command. See
explain for more details.